One of the biggest turn-offs in 3e and 4e for me were, and are, multiple DUNGEON MASTERS GUIDEs and PLAYERS HANDBOOKs. I liked how 3rd edition seemed to be handling it with ... I'm guessing the term is "splatbooks"? - expansions, but not essential.
I didn't care for multiples of the MONSTER MANUAL and by and large the FIEND FOLIO isn't anything to write home about. The minions and in some cases downright fightable critters in DEITIES & DEMIGODS are OK as they sort of make each pantheon more filled out (there are some "can be handled by low/mid-level parties" critters in the ol' D&DG), but monster books I can understand. As you get more creative stuff coming from modules (the aspis in A1 SLAVE PITS OF THE UNDERCITY, for example) and they build in numbers you want to have them in one place, it saves us DMs having to tear our hair out trying to remember where the insert for S4 got to - dammit it has the critical information for the behir!
But DUNGEON MASTERS GUIDE 2 (and indeed 3 PLAYERS HANDBOOKs)...eh, not so much. I think a well-written DUNGEON MASTERS GUIDE should have all you need in it.
That is a thing that needs to Go Away. If you've rewritten the rules to the extent that another 200+ pages are required to grok them, haven't you rewritten your game, by and large? And if they're not new rules, but "just optional" stuff, and you tell your customer base "Oh hey no, no, the game is totally playable with just the three", you're submarining your own product and Big Hasbro Is Watching.
So the multiple iterations of core books, that's something I don't think much of.
...now with that said...
A "domesday" or "omnibus" D&D edition that does support many D&D playstyles...well, it just might need 3 PLAYERS HANDBOOKs! "Splatbooks" might not be enough to cover the customization settings for a guy who wants a full {EDITION} game versus whatever the default settings are.
Much food for thought!