Fantasy gaming with higher level characters is uncommon. Proponents of newer editions proclaim a mid-level "sweet spot" that erodes as spellcasters begin to take over the game, making the other classes obsolete. Is this a problem in OSR style games as well? There are far fewer discussions about high level characters or high level campaigns.
Most of the discussions around the sweet spot focus on the magic user's ability to dominate encounters with damage output - the sweet spot is that transition period where all the character classes are contributing equally to dealing with tactical problems. There's a secondary concern about utility magic obviating the need for a diverse party in the later game too, but today I'm focusing mostly on comparing fighters to magic users in the combat arena. We've bounced around different old school style games so I'm curious how the relationship between the magic user and fighter scales in higher level games across some of these systems. I've always assumed AD&D handled fighters the best, but both ACKS (Adventurer Conquer King) and LOTFP (Lamentations of the Flame Princess) have gone in vastly different directions with their respective approaches to "niche protection".
There are some additional factors to consider. One of our typical adventure sessions might cover 3-4 encounters, plus the potential for wandering monsters. The goal is to avoid another later era phenomenon, the "5 minute work day", where the party rests and retreats after every encounter. So the question becomes whether the magic users have enough gas in the tank to handle the encounters on their own when there's going to be a series of encounters without rests. Do the fighters act primarily as "offensive linemen", keeping the monsters away from the spell casters, while the spell casters do all the heavy lifting? Contrast that role with the fighters in the early game, where most of the tactical encounters are settled through missile and melee combat, and spells are held in reserve to get the party out of jams they can't otherwise handle. Is this a more satisfying style, which is another reason why we see more lower level games than high?
Magic users are fairly similar across editions (except for LOTFP, noted below). The typical high level wizard is going to have enough spell slots to burn a 1st level combat trick and 3rd level X-damage spell in each encounter, supplemented by a 4th or 5th level combat spell in one of the other encounters. Assume average damage 3.5, a 10d6 spell will let the wizard do 35 points per encounter, with magic missiles adding 17.5 points (AD&D style) or 22.5 points (BX style). Obviously, that 35 point X-spell can be devastating as a burst, stripping hit points from a large number of enemies. Still, that establishes our baseline - a wizard that is rationing his or her magic to have a few good volleys for each encounter is easily laying down 50-60 points of damage, and that's not counting the 2nd, 4th, and 5th level spells or access to devices like wands and staves, many of which can alter combat math even if they don't do direct damage.
Assume an average encounter is against a party of eight high level fighters - a real baseline creature - they'll have 50 hit points or so each, close to 400 hit points across the entire group. If they're caught in a magic user burst spell, they'll have a 50% save chance to reduce the 35 point X-damage - we'll just say the magic user strips 26 hit points from each (35 expected damage, reduced by the expected number of successful saves to 26 average). The party will still need to dish out 24 damage to each remaining fighter (and the magic user should be able remove one with magic missiles).
For calculating to hit and damage, I'll assume the fighter has a +1 from strength, is using an appropriate magic sword (we'll say a +2 sword) for an average damage roll of 8, and the opponents are wearing good armor (plate and shield).
BX Fighters don't get any extra attacks per round. They'll hit 70% of the time, doing a meager 5.6 average damage per round (round up to 6?). Trying to wear down those opposing 10th level fighters and their remaining 24 hp each is going to take a long time in BX terms - very grindy. Each BX fighter will need an average of 4 rounds to mop up a single opponent.
The AD&D fighter also has a 70% chance to hit on each attack; the difference is they get an extra attack every other round, so they'll average 9 damage per round instead of 6 damage per round. If you're using weapon specialization, the chance to hit is 75%, and they attack twice per round for an average 16hp damage per round.
The AD&D fighter is significantly more effective than BX - and keep in mind that fighters with high strength will be doing more than a +1 damage bonus (bringing that hp-per-round closer to 18-20hp per round). Unfortunately, the NPC opponents would have 10 extra hit points (AD&D uses a d10 for fighter hit points) so we're still looking at 4 rounds to finish an opponent (or 3 rounds if specialization is in effect). The AD&D fighters clean the floor against monsters with 8-sided die hit points.
The 10th level LOTFP fighter will be hitting only 65% of the time, and they don't even get a damage bonus from strength. They'll average 4.5 damage per round (round up to 5). This fight will be even longer and grindier than the BX version.
Keep in mind that there are no X-damage Magic User spells in LOTFP, and the other characters in the fighter's party (even at 10th level) will only have a pathetic 10-15% chance to hit in combat each round against the heavily armed sample opponents. In the kingdom of the blind, the one-eyed man is king - that's one way to make the fighter seem good. LOTFP obviously doesn't assume the same degree of tactical combat is going to be happening as in other retro clones. In fact, the proposed sample fight would be a mind-numbing affair - at least 10 rounds to kill a single 50 hit point opponent. Best just to summon a demon with the level 1 magic user spell and let the eldritch horror go and fight.
ACKS uses a modified BX chassis for the fighter - the base chance to hit the opponents at level 10 will be 65% of the time, increased to 70% if the fighter has a "fighting style proficiency", which is likely. The ACKS fighter gets a damage bonus based on level, which is +4 at level 10. Thus, our ACKS fighter's average damage is 12 per successful hit, which equates to an average of 8 damage per round (3 rounds to knock out one of the opponents). It's an upgrade from the BX fighter, but it's below the AD&D fighter (and way below the souped-up "weapon specialization fighter" from AD&D's Unearthed Arcana).
However, there are two other factors in ACKS - there's a cleave rule, which means every time one of the opponents is taken out, the fighter gets to tack another 8 average damage on the next one (a down payment on defeating the next guy), and there's a system-supported critical rule (a weapon proficiency) that maxes the damage on a natural 20.
I expected to get to this point and begin a discussion on challenging high level parties - for instance, ways to present difficult tests outside of combat that push the spell casters into selecting powerful utility spells and forcing interesting resource choices. But after seeing how poorly the fighters scale in pretty much all the old school variants, do you really want to run high level tactical challenges where the fighters carry the load? On the contrary, you need to be jamming more magic wands at the casters and let the party continue the asynchronous warfare. A combat featuring the fighters is going to feel like one of those 19-inning baseball games (score tied the whole time 0-0 until the first run is plated)… ZZzzzzz.
From the perspective of peaking early, enjoy those moments of glory while you're level 1 and level 2, fighter players! It won't get any better than when you're holding the front line in plate and shield, and killing off humanoids and bandits every other round or so. It's like high school all over again, where that popular jock guy seems kind of successful now to your teenage eyes, but in a decade or so he'll be the one with the beer gut and the bald spot who never left town, and that brainy nerd from your chemistry class is the next Bill Gates (ie, the 10th level party wizard). Magic users are living the white collar dream.