All this talk about Nikon's 'huge' new Z mount leaves me pretty cold TBH. Sure the F-mount is a bit tight for some fast primes, but how fast do you need to go? When you get below f/1.4, lens size, weight and cost escalate dramatically, and performance takes a hit. Look at Nikon's (manual focus) prototype 58mm f/0.95 Noct

Pointless IMHO. Meanwhile, that massive hole in the front takes up valuable space for everything else - like dual memory card slots, or a bigger battery etc.
Nikon appears to be playing hardball with the new Z-mount protocols. Unlike Sony, they're not releasing them to third-party manufacturers which means they'll have to reverse-engineer them the hard way - as usual. It's clearly a business decision Nikon has made and since we don't know the business impact of that, it's hard to comment.
I don't see a flood of amazing adpaters appearing though, allowing you to fit any DSLR lens to any mirrorless camera, any more than we've seen so far. What's possible in theory seems to turn out (Metabones etc) to be expensive and often with less than optimum performance. Camera manufacturers are in business to make money, and selling their own-brand lenses is a big part of that. They've learned to live with Sigma and Tamron etc, but only because they've had to.
My guess is that Canon's new FF mirrorless will have a new mount. It's a great opportunity to start afresh with something truly future-proof, and since we'll need a mount adapter to use Canon DSLR lenses anyway, it makes no difference what's on the other end.
Canon will be under huge pressure to to reveal 'something' at Photokina next month. It could be anything from a mere statement of intent, to a mock-up in a locked display case, to a pre-production prototype, even the real deal. But since Photokina is only weeks away now and we've heard next to nothing so far, I'd say a mock-up is the best we can hope for, with a promise of more to come next year.
Given the Nikon Z's disappointing AF performance, it seems to be a tougher gig than expected (hats off to Sony). Canon will wait until they're ready, and given their strong market position in both DSLRs and mirrorless M-series, time is on their side at least for the short term.