I think that because of the PHB+1 rule, these rules make more sense integrated with the PHB. Otherwise, they can't be used with all the other sub-classes from other books.
Sooner or later, PHB+1 was always going to be untenable, so I don't really see that as plausible reasoning. It's unlikely they get integrated because that does make this 5.5 (or like, 5.1, really but people will call it 5.5), in that everyone even Beyond people (like me) will have their expensive hard-copy PHB partially invalidated.
The question about Xanathar's is fair. From 10/10/2016-6/5/2017, UA had 102 pages of material meant for XGtE: a lot of that did not make it into the book (Mass Combat rules, many of the Subclass options), and some was revisions of earlier stages of the playtest. XGtE is 192 pages long, so they tested the equivalent to half the length of the book: counting pages, ~75 pages in the final book are from UA directly. Further UA releases may shed further light.
They already revived Greyahwk for publication this year: returning in full force in a Setting book as a gonzo "Sword & Sorcery" genre booster would make sense, but it is just one possibility.
75 vs 34, interesting - I guess if we see a whole lot more, a Xanathars becomes more likely, but if we don't some kind of setting book does.
Re: "Gonzo Sword and Sorcery", the big problem is,
nothing about Greyhawk is gonzo (yeah not even expedition to the Barrier Peaks - it's actually totally MoR by modern standards and this is 2019 so modern standards apply). I know you're going to try and come up with examples, but no. Metal, maybe. Some (not much) stuff in Greyhawk is kinda moderately metal, like sub-Metallica levels of Metal, not really that metal at all. Dad-metal. But gonzo? Gonzo is a high bar. It makes me think of things like Over The Edge, Dungeon Crawl Classics, Underground, SLA Industries. Even we lower the bar to the say, Dark Sun and the wackier end of the Planescape spectrum (which to me don't reach "gonzo" but I guess I could sort of squint and see it), that's not just Greyhawk. Greyhawk is basically someone's dad. Sure, you can give him a Keith Flint-from-the-Prodigy haircut to hid his bald spot, give him some piercings and tats, some OTT eye-make-up. But he's still someone's dad. It's fundamentally a very generic setting full of very generic stuff. Amping up a couple of "Metal" bits and using some edgier-than-usual artwork will not fix that (plus has WotC got the balls to do that with artwork? I give 50/50 at best).
Now, maybe you could fix that, but you'd basically be writing an entirely new setting, riffing off Greyhawk. As far as I know, neither TSR nor WotC has ever dared to do something like that with any D&D setting. The closest was perhaps the 4E FR, and much as I like 4E, that was a bloody car crash. I do think there is the potential for a cool, "Greyhawk-inspired", very metal, perhaps near-gonzo setting, but that requires you to burn a lot of Greyhawk stuff, and probably to piss off an awful lot of Greyhawk fans.
Further, even for a setting book, I don't think you could get away with this material. Not only are many of the subclasses not "Greyhawk-friendly" (at least for the "old" Greyhawk - they don't draw from Greyhawk ideas or traditions), but them plus this stuff just isn't enough. You'd need a class, and the only likely class soon after Artificer is Psion (and even that is questionable, sadly), and if you're making Psion a class in a Greyhawk book you're nuts.
Dark Sun is somewhat more plausible, but again many of the subclasses aren't good fits (giant-viking-themed warrior for example).
Planescape or a "Manual of the Planes" could probably stitch together the subclasses, though, so perhaps that's the most likely if we're going with settings.
