Netflix won't allow TV makers to make their own Netflix app, they must use Netflix proprietary code as-is, and it's Netflix code that download chunks and forwards them to playback, while TV handles the secure video decoding, so TV can't "control the entire stack"
Also, tv boxes are allowed to playback 4k Netflix just fine without controlling the screen.
All the Netflix shows you can download on the internet do include the 4k versions.
Maybe they'll obsolete older 4k devices that are deemed less protected (though I could probably point them "a few" security flaws to Netflix-endorsed 4k devices so that every is on par), but I doubt it. I don't think anyone ever done that (720p has always been fine without HDCP, 1080p with only HDCP1, before that macrovision was non-breaking) [1], and I doubt Netflix would want to push that badly towards e-waste.
[1] There has been few cases of some devices being revoked, like Nexus 6, but it was usually long after their shelf life anyway