I had Xbone pegged for 60-65m lifetime but that's starting too look a little over-optimistic.
One thing's for sure, they need to drop the "Xbox One" branding and pick a new name for their next system, the words "Xbox One" have come to be associated with inferiority. Even after all these years they've been unable to outrun the shadow of 2013's monumental blunders.
The next xbox will be coupled in harmony with Windows 10. I could see this affecting the name. Xbox is more about services than being focused on the console, so we'll see.
My prediction was 45 to 50 mil max by the end of gen. Ive been out of the loop on VG Chartz most of 2018, so my prediction was from 2017.
I think the Windows thing actually hurts the chances of the XB2 or XBW(indows.) I mean, if you can get all of their games on PC, why not upgrade your PC for about as much as the, probably, $499 Scarlet will cost? Granted it would probably be closer to the base XB2 in power, but it'll still play all of their games. I know there are some people, like myself, who just prefer console gaming. Still, if you only get a XB2, it seems a little pointless. A PS5+PC, like the PS4+PC of this gen, is just the way to go to get pretty much everything. Just need a Switch to get everything. I just don't see many people going XB2+PC or XB2+PC+Switch.
And it looks like your prediction is going to be close, maybe even spot on. I think it will finish just north of 50M, maybe 52M, but it stops there. This year doesn't look to be off to a good start, at least according to Amazon. It's only SKU on the Top 100 for the year is at #95, and is $229 with a 1TB HDD and Minecraft with some bundled DLC. That's $70 cheaper than the standalone PS4, which is at #49. Even the $170 more expensive Pro is sitting at #82.