On principle I dislike level limits. On the other hand, the nature of skill development is that the more advanced one becomes, the more advancement slows. There's also the limiting factor of time and aging. So a "realistic" D&D would actually involve less power increases the higher level one becomes - or at least far slower advancement to new advancements.
That said, D&D is a game and doesn't need to model itself after the real world (ahem). I personally see no reason why a new or alternate epic capstone system wouldn't be a good thing, and I would be very interested in seeing it developed. Furthermore, I think it should include something new, that isn't just "more of the same, but more" -- meaning, make it truly gonzo. Which brings me to the idea of immortal play.
If 17-20th level characters are "Masters of the World" with "superheroic capabilities," then (say) 21-25 would be something beyond that, and 26-30 another tier up. So here's a basic structure:
21-24: Epic hero. A natural extension to 17-20, but moreso - and with epic boons and such. This would fill out the extent of mortal power. It might be similar to the current system, but formed into levels and with increases to proficiency, more spells, etc.
25-29: Quasi-deity. On the path to immortality. Among the most powerful beings in the campaign world, able to go one-on-one vs. the most powerful monsters and fiendish lords. To get to the next tier, some kind of "immortal elixir" is required, and/or granted by the gods.
30-39: Demigod. At level 30, true "ascension" occurs and immortality is attained. The PC enters the pantheons of gods and becomes effectively immortal, although without the power to influence masses (yet).
40-49: Lesser god. Play becomes more meta -- conflicts between gods, cosmic venues, etc. Able to incarnate as an avatar, yet still with a true form beyond the Prime Material Plane.
50 (level-less): Greater god. World-makers. Play is almost akin to world-building, but with the capacity to incarnate in avataric form.
Or something like that. Obviously the higher the tier, the more different and only sketched out the rules would become. Most epic campaigns would like remain below 30th, but elements of higher tier could be employed.