Now that MS has acknowledged that they can't beat Sony at Sony's game because of entrenched digital ecosystems and few new entrants to the market, they only really have two reasons to bring out new hardware: GP and dedicated fans that are part of the Xbox ecosystem (mostly which are the same customers at this point).
They don't make much money if someone doesn't subscribe to GP, so I'm betting their focus is to sell new hardware to GP subscribers rather than the other way around. Thus, I agree with the notion that they will try to sell as much as possible through digital storefronts, especially in Europe.
I'm thinking 2027 now, as that's what the rumor mill and AMD's time table are suggesting. Unlike some people around here, I don't think they need to worry about Sony's time table anymore. Their hardware is just about maintaining and expanding GP and not about attracting PS owners anymore. 30+ dev teams can fill in any software gaps and besides, we're looking at 2-3 years of cross gen anyway.
The idea of it being a PC in a box is great as that would allow for higher end versions for enthusiasts from OEMs, but the base version can't be more than $100 more to make this happen. In other words, if the base "console" Xbox Prime would have been $599, then I think making a PC compatible Xbox Prime is worth it for $699 or less. The brilliant thing about the PC Xbox Prime is that Sony ports will work on it. Sony couldn't bring a title to PC without it working on the Xbox Prime. Sony would be "forced" into multi-platform.
Xbox Marketing: "Get an Xbox Prime and GP for amazing value, and play The Last of Us and Spider-Man if you want. We're all one big happy family now."