On the one hand, I personally wouldn't miss high-level info in core, because I don't usually play at those levels.
On the other hand, it's nice to have that info there when you need it, without getting another book. Plus, high-level has always been core; that sort of gonzo D&D is part of - possibly even the heart of - D&D as a genre.
I get the not wanting to go to another book... but it feels like we do anyway with lots of spells/subclasses/etc...anyway with Xanathar, Tasha, Mord Multi, and who knows what coming down the road.
Say everything got shoved in the DMG that would go there, what are the feelings of:
PHB + MM + Xanathar + Tasha + MordMulti
PHB* Low + MM* Low + PHB* High + MM* High
where the * indicates it has the relevant stuff from Xanathar + Tasha + MordMulti too? At least we'd know which book to look the spells up in by level. And for monsters, I'd be happy with one super fat book instead of two thin ones).
(As @R_J_K75 note above, it's not like I imagine they'll deviate from the way they've been doing it in any case).