From Digitaltrends:
Many reasons exist to upgrade to Windows 10, and for Windows XP and Vista users — which according to some data represent a bit more than 10 percent of all PC users — perhaps the biggest reason is for the night-and-day differences in support and security that Windows 10 provides. Google just offered another reason update to Windows, specifically that Gmail will reduce support for Windows XP and Vista, as Google announced on the G Suite blog.
While those users will still be able to access their Gmail messages, they will be doing so with the much less robust HTML version as early as December. The Windows version is actually a secondary cause of the reduction in functionality. More specifically, Google will be shifting all users running Chrome Browser v53 or below and it just so happens that the latest Chrome version supported on Windows XP and Vista is v49.
[...]
Google’s specific statement regarding the reduced functionality is as follows: “Gmail will continue to function on Chrome Browser v53 and below through the end of the year. Users who remain on Chrome v53 and below could be redirected to the basic HTML version of Gmail as early as Dec 2017.”
As a Windows 7 user who's currently running Chrome v56, I can say categorically that this does not affect users of Win7 and up (unlike some reports that you may have seen), so 47.2% of Windows users will not be affected by this. Chrome, however, currently runs on 57.9% of all PCs, so there's definitely some significant portion of PC's Chrome user base that will be affected by this. It will be interesting to see whether those users stick with Chrome and accept the reduced Gmail functionality, or switch to Firefox to get a better experience.
The odds of them dumping Gmail, naturally, I rate at basically zero. Switching your email address is such a pain in the ass to do, I can't imagine that anyone will bother with it, especially since the main cloud-based email alternative is probably Hotmail, which Microsoft is presumably keen to marry to both Bing and Edge.