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.