I do wonder whether you did much testing (or had it done by someone else) with applications using multiple swapchains for this patch. In particular, MS Office and various WPF applications come to mind.
I wouldn't call it extensive testing but I've been using various Office 2016 applications with some version of this patch over time and I don't recall encountering issues related to it.
An older version of the patch has also been included in a certain CrossOver branch which, as far as I'm aware, sees heavy use of Office applications.
It's probably not as much testing as we'd like but I am sufficiently confident that, if not specifically this particular version of the patch, this change in principle should be fine.
Finally, there's the question of whether less than a month before the code freeze is the right time to make this kind of fundamental change. I suppose we've already done worse this cycle, so perhaps it's fine.
That's a fair point. I'd be inclined to say "what's one more big-ish change anyway :D" but yeah, it's up to you.