To continue with the question raised in yesterday's post ("Why is it OK for spell-casters to be able to overcome increasingly difficult challenges but not OK for mundane characters to be able to overcome the same challenges?"): the problem with most proposals I've seen to allow mundane abilities to scale with level in the same way that spell power scales with level is that they often are not scaled the same way. I've seen proposals for damage to scale with level, armor class to scale with level, speed to scale with level, Hear Noise or ability to surprise, jumping distance, distance fallen before taking damage... in fact, the monk class serves as one example combining many of these. But the monk class, and many of the other proposals, talk about scaling multiple or even all at-will physical abilities.
Sure, a 5th level Magic-User can use a Fly spell to cross an enormous chasm: once. After having spent a minimum of 8,000 gold pieces to learn the spell. And at the cost of not being able to cast Fireball or another 3rd level spell that adventure.
The super-Fighter, on the other hand, is supposed to jump a similar chasm. And then another similar chasm. And another, as many as it takes. Plus, have better damage, better armor, better dodge, better ability to open stuck doors, better everything? At will?
The two classes are not comparable.
D&D designs that did decide to go the route of scaled abilities chose not to make a broad number of them available at will, precisely because the designers knew that the unlimited super-fighter wasn't comparable to the more limited Magic-User design. Instead, they chose either "improve one ability at a time" (feats) or a mix of at-will and more limited powers for every class. But of course, those designs meet specific play needs.
Some other house rule approaches added some kind of fatigue or action point system, but that adds extra book-keeping. Not everyone wants that.
And some people changed the Magic-User to a super-magic-user, removing spell memorization entirely in an attempt to keep them comparable to unlimited super-fighters. Again, a design more suited to players who want high action and cinematic scenes instead of "how do we get over this chasm?" moments.