You don't need as much RAM once you dial down the graphics settings. In a 1080p game you don't really need 4K textures for example, so you can save a bunch of memory there.
Once devs start considering 16GB the norm, they also start making less efforts to fit games in lower RAM size. Sure, scaling down graphics you save a lot of memory, and initially 10GB won't be a problem, actually for cross-gen games that must run on old gen too, with its 8GB, they will never be a problem, but they could become later for current gen-only games and even worse for current and next gen cross-gen games. 12GB is 50% larger than last gen RAM, but 10GB is just 25% larger. With better CPU and GPU even just those additional 2GB can make us feel the generation leap at the beginning of this gen, and graphics scalability will keep problems away from graphics on top end models too until the end of this gen life, but later it could arrive the moment when having to run in the base model's 10GB will force to underuse the CPU, as CPU usage scales down much less downscaling graphics, particularly levels size and its logic, not graphic detail, so game world management related to it, and gameplay don't scale down at all, neither do physics, except particle systems, that are managed by the GPU, anyway, as the CPU isn't parallel enough to do it efficiently and fast.
Anyhow, large game worlds with lots of details not limited to graphics, but also regarding items interactivity, will eventually become the worst enemy of systems with smaller main RAM, and most probably only exclusives will manage to keep the problem away for the whole gen.
@DonFerrari This is also at least a partial answer to your point, I totally agree that scalability will keep graphics out of troubles, the problem will be the RAM needed by the CPU for all the things that can't be scaled down.