Won't be a new generation until there's Scorpio exclusive titles.
MS is done with generations. Four or five years from the release of the X1X, we'll get another new console that will likely play all the same games that we can play on the X1 now, and the new games that come out then will likely be compatible with older hardware, at least to some degree.
In 2017 and moving forward, what the consoles can and can't play has really nothing to do with what "generation" designation it falls under, just like how I can play newer computer games on (reasonably) old hardware, as well as play older games on my newer hardware. PC gamers don't worry about hard breaks and generation designations, and now consoles are moving into the same boat. The only way this changes is if the X1X completely bombs to a catastrophic degree, and at least based on preorders that doesn't seem likely to happen (that said, the X1 sold well out of the gate too, and look how that would up).
Eventually we'll start to get new games that don't work on the original X1 console, but there will be no hyped up statement from Microsoft saying "this is it, from now on games only work on the X1X" or whatever. Eventually the games that work on the original console will just slow to a trickle and then stop.