WheresMyD20
First Post
I haven't played 1e at high levels, but I have played 2e and that's not a lot different. And there are certainly ways -- and somewhat trivial ways at that -- for a wizard to be almost immune to mundane attacks at that point. Spells can take the wizard beyond any effective attack range (fly and related spells), make it impossible for mundane characters to target the wizard (invisibility and related spells), make physical attacks ineffective (stoneskin, protection from normal missiles, etc.).
At 20th level, both fighters and M-Us have ways of dealing with mundane attacks. Fighters are so well armored and have so many hit points at that level that the vast majority of mundane attacks miss and those that hit have to chip away at a large hit point pool. A M-U needs powerful defensive magic to protect his 37 hit points (on average) and bad AC.
Invisibility is of limited usefulness in high-level 1e. There's a % chance based on level and Int that anyone can - without magical help - effectively see through invisibility (DMG p.60).
Stoneskin (which actually isn't part of base 1e - it's in Unearthed Arcana) is certainly useful, but not usually to the same degree as a fighter's armor and hit points. Protection from normal missiles is not very useful at 20th level - normal arrows and sling stones aren't commonly encountered at that level and usually don't pose much threat.
Fly is definitely useful - if the magic-user is outdoors. The rate of flying is only 12", which is the same as unencumbered ground speed - and half that if ascending, so it's not a particularly rapid escape or evasion.
Remember that a single area of effect attack like a fireball, lightning bolt, dragon's breath, etc. can often times kill a high level M-U even if he makes his saving throw. Fighters can usually weather a few of those.
All of this isn't to say that fighters are more powerful than M-U's at high levels in 1e or that they're even equal. The point is that fighters are still quite useful at high levels in 1e and M-U's aren't all-powerful. In 1e, there is room for both fighters and M-U's in a party and a party that is lacking either fighters or M-U's is usually in for a rough time. (Let's not even get into clerics who are quite vital too - you need a good mix of classes)
One of the great improvements from 2e to 3.x was that it made high-level play merely time-consuming for the DM and somewhat unbalanced (in favor of full casters, especially druids, wizards who weren't blasters, and clerics who weren't healers) instead of nearly completely unplayable.
There's where I'd have to disagree strongly. I played 1e and 3e at high levels and found 1e to be quite playable (we played at levels 20+ for literally years) and 3e to be terrible at those levels (the saving throw system in particular is a mess at high levels).