I'll ask again: If the PCs always have to worry about being killed by any schlep with a weapon in his hand, what's the point of levelling up?
Again, doubling, tripling, and quadrupling Hit Points is not the one and only way to power up characters. In a high-AC, moderate-hp system, a high-level Fighter wouldn't worry about "any schlep with a weapon in his hand," because he'd strike that "schlep" down, same as always. And, realistically, no "schlep" would approach Sir Lancelot to see if he might be the guy to score a lucky crit (and still die the next round).
We're not talking about reducing the power level; we're talking about changing the mechanics for high power. In a high-AC, moderate-hp system, a high-level Fighter can wade into battle against hordes of spear-carriers, and he'll kill them as spectacularly as always. He'll Cleave and Whirlwind Attack them the same as always. Instead of getting whittled down though, getting hit most of the time, but only taking 5% of his Hit Points per shot, he'll get hit much, much less often -- but those hits might take 20% of his Hit Points at a time.
In much the same way that infrequent leveling up is more exciting than incremental improvement, fewer hits that do more damage are more interesting than consistently taking a few points of damage for twenty rounds in a row.
And it makes more sense, since hits can be hits, and misses can be misses, and we don't need a lot of hand-waving to excuse mechanical strangeness.