What makes a high level fighter relevant in a 1st edition game? Far longer than they maintain their relevance in later editions of Dungeons and Dragons?
In first edition, fighters always get all their attacks, even when they run across the room. There are some minor variations between the various flavors of AD&D, First edition fighters get many attacks vs. less than 1 HD creatures. They also may not select their opponent when fighting more than one person. In Hackmaster fighters may not split their attacks, but they get follow-through damage.
In first edition, fighters are hard to hit. This is even more the case in Hackmaster where armor works as damage reduction. They can take a pounding, which is important, because of something that leads us to point three.
In first edition, magic-users are delicate white porcelain mice that if someone looks cross-eyed in their direction will have their spells fail. It is true that there are several spells that in most cases they can get off if an opponent has not yet closed into melee - but in melee, even casting a one segment spell has a 50/50 shot of being cast without being interrupted.
Experience point tables were logarithmic and not equal for every class.
A fighter with 10,000 ep? Level 4. A Magic-User with 10,000 ep? level 3. The magic-user will always be leveling behind the fighter.
Finally, balance is irrelevant. Sure the wizard will have all sorts of powerful spells eventually. The fighter will have an army (and most likely a fortress). None of that matters at all, really, because the world is dangerous and not level appropriate. Also, controls on spells are a little strict. Teleport carries a non-trivial risk of death upon use, Haste requires a system shock roll to survive it's casting - the world is a harsher and more interesting place.
The real point here, is that the vast majority of the second, and more importantly, third edition changes that were made to have the game make more sense, in fact destroyed a lot of the fun and balance present in the older games. I believe this is a large part of why the OSR exists.
I find when we play Hackmaster, players are concerned with world goals - let's defend this town, let's train these people, let's build this wall, let's insure nothing takes up residence in this dungeon. When we play Pathfinder (3.5) players are concerned with character goals - I need another +1 to hit, How can I get my intelligence up to raise the DC of my spells, what's my next feat going to be?
I know which I'd rather be dealing with.