This is the real answer. Vram is largely dependent on the resolution you're running, and at 1080p 8gb vram is fine. People who want 20GB vram are probably going to build their own machines anyways, the steam machine is meant to be a console replacement to my understanding.

I'd argue that 1080p gaming is also perfectly fine. These days most games have split the UI/window resolution from the game resolution. So you can have 4k sharp text and UI, while the actual game runs at 75%/50% resolution and you largely can't tell the difference while sitting on the couch.

Is it dependent on the resolution your running, or is it the size of all textures that need to be cached in RAM? The amount of data needed to framebuffer 1080p vs 4K isn't that great