I can't speak for everyone, but 4E for me had a lot of class bloat. I personally disliked the approach of making a class for every source/role pair, and I disliked powers being by class instead of being largely shared (for example, look at how many spells in 3E/5E are shared between classes).
Remaking a 4E retroclone would be am exercise in game design for me. A way to clean up the player side of 4E while keeping the (what I see as) perfection of the DM side of the game.
But why would condensing things into 20 levels help any of that? If anything it would make the system
more bloated, as you'd be needing to make decisions every single level even if you DID cut things out. You'd have 20 much busier levels! Why not just...
not do Epic at all? It seems like a lot less work to just say "I'm not going to bother re-doing Epic tier right now" and instead simplify only 4e's first 20 levels.
As for the source/role thing, frankly I think almost everyone who complains about that makes a ton more out of it than it really is. Because it
wasn't "every source/role pair must have one class." There never was a proper Martial controller--Essentials back-ported a semi-Martial semi-Controller through a ranger subclass, but there was never a formal class for it. Divine and Arcane got two different leader classes, and they're the only ones (though Primal got a leader subclass for Druid). Primal got two Controllers (Druid and Seeker), but Arcane--theoretically the "controller source"--didn't. Martial
started with two different striker classes. Shadow never got more than two dedicated classes, both strikers (Assassin and Vampire). Elemental
never got dedicated classes at all.
There never was a commitment to "filling" every role/class pair. Even the Psionic classes had precedent already (Battlemind = Psychic Warrior, and Ardent and Psion already existed from 3e.)
Besides,
several really cool combos with their own new flavor and ideas came out of this supposed "bloat." The Warden, 4e-style Shaman (that is, with a spirit pet), Avenger, Invoker,
Warlord. All great ideas that came, not from "f**k, we HAVE to fill our grid!" thinking, but from "well...what WOULD a Divine Striker[/Martial Leader/etc.] look like? What would it
do? Why would it exist?" Incarnate forces of nature, communers-with-spirits, church Internal Affairs, Moses-esque plaguebringers/sea-parters, fantastical drill sergeants. All genuine and (IMO at least) exciting fluff evolving from asking what ways the classic sources of strength and skill (the gods, nature, esoterica, grit, the mind, darkness, raw elements) could be channeled into the things adventuers
do.
I get that all of this is very very much a matter of aesthetics, of taste and preference, so it's not exactly something that can be argued. But there really never was a "make a class for every box" attitude. Almost all of the classes that appeared in 4e were either well-precedented (Psionics being the poster child), or had real and novel ideas going into why they would exist. Honestly, the
only one I can think of that doesn't fit that mold is Vampire!