I think the reason that epic play hasn't been well supported in D&D is less because it doesn't work, and more because people don't know what to do with it. And, of course, there's the simple logistical fact of numbers: There are more low level campaigns because the vast majority of campaigns start at low levels, and most campaigns eventually peter out.
So here's a thought, to follow-up on my previous post: make epic play truly epic, exploring new venues of play. Don't just make it bigger and badder; make it mythic and cosmic. Expand the notion of what D&D to incorporate cosmological elements.
A so-called epic book could really be about this "new" kind of play. A big 320-page book could be in three parts:
1. Epic characters - levels 21 and up.
2. Epic campaigns - a toolbox on how to run an epic campaign.
3. Epic monsters - deities, demigods, and more.
The book could encourage cosmological-customization, and even eventually offer new worlds to explore. An example might be:
The world is ruled by a pantheon of deities in a tiered hierarchy, what we could call "Celestials" that are worshipped in different cultures by different names, but essentially an ascended order of immortal beings that governs the known cosmos. They "wait" for heroes to emerge to join the ranks, becoming part of the Celestial Hierarchy that abides over and guides not only the worlds, but a family of worlds under their guidance. The worlds are seen by them as "cradles of the gods" - their task to facilitate the evolution of individuals towards Ascension (attaining immortality).
In this system, when a character reaches 20th level they become a candidate for Ascension. Epic rules continue for four or five more levels, when they have to prove their worth. Upon attaining 25th level they can Ascend and become a Celestial Paragon: akin to a quasi-deity. Every five (or ten) levels brings them up a rank in the Celestial Hierarchy, like so:
25: Celestial Paragon (quasi-deity)
30: Celestial Paragon (lesser demigod)
35: Celestial Paragon (greater demigod)
This first triad would be more traditional epic play, but expanded into greater quests, both within and beyond the world. Celestial Paragons guide and influence nations, perhaps under the governance of a deity.
40: Celestial Deity (Lesser god)
60: Celestial Deity (Intermediate god)
80: Celestial Deity (Greater god)
The second triad are true gods of the world: they guide through influence, leaving active work to the Celestial Paragons, but caretaking the world and its peoples.
100: Star Celestial (Demiurge; World-Maker)
500: Galactic Celestial (Galaxy-Maker)
1000: Universal Celestial (Cosmos-Maker)
The third triad have influence beyond the world. Each Star Celestial/Demiurge is the creator god of a world; upon attaining that level, the deity is given the task of creating a world and its system; they are the star or sun, the center of system, and game play would expand to billions of years. The Galactic Celestials govern the Star Celestials, and the Universal Celestial is the supreme being of an entire cosmos.
Or something like that. Obviously the vast majority of players would enver play beyond the Celestial Paragon triad, but such a framework is just one example (written ad hoc, so forgive the roughness of it) that could provide for different types of game play.