All Xbox One games that you already own, including Xbox 360 games, will work on Project Scorpio. All PlayStation 4 games that you already own will work on PlayStation 4 Neo. Any games released on those new systems will also work on the older versions.
This is a crucial component of the death of normal game-console cycles. It means that console makers like Sony and Microsoft could move to a more smartphone-like model, with upgrades coming out every few years. It means that, instead of a PlayStation 5, Sony puts out several versions of the PlayStation 4 that eventually evolve into the PlayStation 5.
Whatever Nintendo does, we’re already moving toward a new paradigm where the traditional agreement between console makers and buyers is going to change dramatically.