Center Stage and all the other new features look super cool as usual, but iCloud Time Machine would be SO GREAT. I have used services like Backblaze before, and they're fine I guess, but third parties don't give me as much peace of mind as built in services. I suppose there's also something to be said for not having all your everything invested into one ecosystem, but I'm not sure where else I would trust my data, not that there's anything in there anyone would care about.
My suspicion is that Apple is aware that there are a lot of people that use Time Machine and may even consider it mission-critical if they value the ability to easily move from Mac to Mac or restore a destroyed Mac from a hard drive. I would guess it's a much more complicated backup than an iOS backup, and would probably be locked to the premium iCloud bundle.