The problem is when that 100 damage is just as ineffective as the 10d6 fireball. Keep in mind that a monster with full HP is just as dangerous as a monster with 1hp. If you can't one-shot a monster, then the monster still gets to attack you.
And, you can't build a fighter-type (tangent, I have no idea if we're talking the class fighter or the archetype more generally at this point, it seemed like the latter) that can survive a single round or two of combat against an opponent? Is that the ordinary state of affairs?
More generally, this logic seems ... exaggerated, I think. We can acknowledge that Glitterdust and Solid Fog are effective spells, right? They substantially hamper enemies. But, they don't actually eliminate them. Being able to quickly dispatch them is still actually necessary. A 50% miss chance is a huge advantage, but it isn't an "I win" button.
What I'm going for is this. If the definition of "effective" is reduce enemies so that they can be coup de graced with a rock, then things are getting way overstated. To give a sense of it, only a handful of the grade A ultra God spells (per TreantMonk, for instance) would not pass muster.
And, going back to the context of these comments, was it the case that AD&D fighters/paladins/etc. were one-shotting baddies left and right? It's been a long, long time for me, but I have no recollection of that being the case at all. I feel like I'd recall if all AD&D fighters were the equivalent of a cracked out Hood. Likewise, I don't
think that enemies are a whole lot more lethal than they were in AD&D -- I seem to recall a ton of save-or-die types of abilities for Team Monster. But, this is based on dim memories.
I'm not really questioning caster dominance in this post. I'm just asking why increasing hit points, while handing melee'ers the tools to do substantial damage, even relative to their magical counterparts, exacerbates rather than mitigates it. At best, it mitigates it b/c the casters are doing relatively less damage. At worst, it's a wash b/c the casters can adopt the same tactics, and probably do them better. But, that brings us back to the niche protection problem. The bigger hp numbers don't seem to be a culprit at all.