Netflix: https://help.netflix.com/en/node/23742 - 720p on Chrome and Firefox, 1080p on IE and Safari and 4K requires Microsoft Edge, a HDCP2.2 display, a 7th generation Intel Core CPU and Windows updates.
Amazon: https://www.amazon.com/gp/help/customer/display.html?nodeId=... - You can do 1080p on pretty much any browser but 4K requires a 2nd generation Amazon Fire TV and a HDCP2.2 display.
In the future these values will go up of course, but the difference between restricting HD users to SD and 4K users to HD is quite different.
That said, YouTube have foisted some odious crap (cough random unsubscibe cough Google+ cough Takedown bullshit cough) on creators and subscribers, and yet here we are. Still using YouTube.
The cost of starting and acquiring users for and running a viable YouTube alternative is just too damn high. Unless Amazon do it, it's not happening. Vimeo is an also-ran despite.
People would just upgrade their shit and/or fall in line, or suffer shitty quality.
That's a sinking ship. That's the sort of thing that allows Facebook into the world despite MySpace already existing. The sort of thing that allows Google to elbow its way into the #1 spot past the Altavistas, Hotbots and the Jeeves. A new site would come out of nowhere that doesn't have that restriction.