I don't think we'll see an edition referred to by WotC as 6th edition. Certainly not as any kind official terminology.
5E is huge, and what's more, it's huge with a lot of people who didn't play OD&D, or 1E through 4E, or played one of them way back when (remembering that even, say, 2000-2004 is "way back when" now, as much as it was the future when I first started posting on Eric Noah's old black board), and only just recently came back. So most of the playerbase isn't excited by or keen on an edition change. They're keen on just "playing D&D". That means continuity and that means backwards compatibility of a strong kind.
I expect what we may actually see is a new-look rulebook and so on in 2024, with some rules changes/updates/clarifications backed into the text, new art, new formatting, and so on. Possibly a new Ranger. But I don't think the changes will be much, if at all, bigger than 3E to 3.5E. So that will effectively be 5.5E. There may well be extra rules, especially optional ones, which you can bolt on, but fundamental changes? Nah.
I guess I sort of agree with Zardnaar in that an actual 6E, a genuine edition-change level of change, will not happen until/unless 5E sales significantly decline. And I think WotC will actually be even more sensitive than that. I think they will wait until people actually seem bored with 5E. Not just not buying it because of saturation. I think saturation will hit years before 6E comes out. I think they'll wait for people to stop playing because of actual boredom and so on, start missing D&D, and then bring out a 6E. And this time there's no doubt they'll fully integrate online stuff, and be ready to go with all that, themselves, for maximum profit, day 1. And I would expect that to contain significant and fundamental rules-changes. But I think we're looking at the other end of the 2020s at the earliest - before 2026 definitely not. 2028-2030? Maybe. They can appeal to the nostalgia of 40-somethings we started with 3E as teenagers and now have kids and stuff... (yes, we will be that old - 15 in 2001 would make you 44 in 2030).
But they probably still won't call it 6E. I'm sure some devious marketing team will come up with something to replace that, and make it clear "this is your daddy's D&D, but also your D&D".
XGtE is nothing like a PHB2 would be.
Really? The material is more or less exactly what I'd expect in a PHB2. I suspect your definition of PHB2 is highly specific and you might want to explain to other people what it is.