Both consoles will use this GPU, it's the best value one, people are fooling themselve unless they actually believe MS wants to suicide launcing a more expensive console or at a later date, It's Gies,the man who said the X1 was going to be more powerful that caused this rumor.
Precisely put, MS has spent a lot on boosting the XB1 and trying to make it viable along the PS4, that included handing a tonne of games for free, selling the XB1 way below what they intended. With a smaller userbase we also have less game sales, less people on LIVE, so it's sort of a domino effect of problems for them.
The biggest issue for MS right now, is that many of it's studio acquisitions and first party studios are branching out or closing down, many games are being cancelled. Many games are in limbo and many are underperforming. Games like Halo5 and Quantum Break did much worse than they anticipated, so is the popularity of their first party games, it's dwindling swiftly and the studios at their disposal are not that great. I think that's the biggest concern for MS, hence why they're trying to branch out to Windows 10/PC to target a bigger demographic for their wares.
I said this to say; since MS have spent so much on R&D for the XB1, we're talking about Kinect 2.0, their controller, the new elite controller... This is a lot of money spent that's not making it's way back to them in a hurry, yet they had to spend so much more marketing dollars, giving away so many games and massively reduce the XB1 s price to gain sales/market share at the second holiday season....We can all imagine how much they've spent trying to get the XB1 on track (and that includes all those tweaks and give-backs in the SDK).....all of this certainly adds on....
So yes, how is it that MS will take on a massive chance on an iterative console, bring on Zen + Vega, most likely to be sold at $5-600 or eat yet again a massive lost at $350-400? Investors and shareholders would be furious, because there's no guarantee that such a venture would be successful, especially when you consider that MS first party studios are not very strong and their rep not that great currently. Having a more powerful console is a huge gamble for MS, because if it fails, it will be a huge money pit.
On the flip, having a more powerful console is less of a gamble for Sony because they possess the mindshare and first party studios, granted (things can change), but people thinking MS's pockets are endless and they can just get the most powerful console out with no thought of what else they need to make that work or no thought of possible failure are clearly not seeing the bigger picture. If all it takes was to put the most powerful hardware out, then many prior console generations would have ended up differently