I think official dnd should include high levels (say, levels 11-20) but that all that info should be in a PHB2 for the 5% of players that actually want it. Such a book would have space for legit high level options, like bringing back prestige classes, high level feats, more high level spells. The core books, however, would focus on giving 95% of players the game they actually play.
The problem I foresee in this is that they two materials still have to be mechanically married to each other*, but the developers of the lower-level stuff will have little reason to design to ensure that the upper level stuff works well. That seems to have happened with 3.0's
Epic Level Handbook. Not an insurmountable problem, but a challenge.
*at least or else there will be a lot of mechanical dissonance when people hit the cut-off.
Interesting anecdote for level caps. Years ago I asked Frank Mentzer if he could change one thing about BECMI, what would it be. His answer? Cap levels at level 20. Obviously level 20 is higher than level 10, but the core reasons are the same: it gets messy at higher levels and fewer people play the game at higher levels so you have a diminishing return on time+effort+money spent creating that content vs. players who play that.
But to the person who mentioned this upthread, even though most players don't play past level 10, seeing there is nice. Personally I would have capped core at level 15 and done a high level campaign supplement later. Cook and Marsh did it right with the Expert set level limit, IMO.
It's worth mentioning that Cook and Marsh made clear on p. X8 that they expected there to be follow-up material taking the levels up to 36.
I think it's very hard not to do this. I know EGG supposedly initially mostly wanted high-levels (especially upper level spells) for enemy archmages and the like, but the instant you put them, artifacts, and epic monsters like beholders and elder dragons into the game, people want to play around with that stuff.
I don't think there's a right answer. However, if you are going to include that stuff, and expect people to actually play it, it really behooves you to make the system work well at that level, and I don't think they ever did. Mentzer put a lot of stuff in the game in his expansion from B/X -- rulership, war, and tournament rules; planar travel; description of the game universe, etc. -- but most of it didn't really need levels (except maybe 'name level and past' although even that is often arbitrary). Re-building the upper levels to make a more mechanically cohesive game would have been do-able, it's just not really been done with D&D.
Or you can go the
Godbound route and still play epic characters, but just reset the mechanical scale back down to not unlike low-level D&D characters (just redefining the in-fiction level scale of the units, etc.).