It is by no means a clean or modern solution (I agree, go with Youtube etc or provide a transcoding js player), but about a couple decades ago many sites would simply link to several resolutions of a video, allowing the user to play either a low, medium or high res version as per their current data connection. So that is one option that sidesteps adaptive bitrate players as well as a large CDN.

As has been pointed out above, having some kinds of video downrated to be streamable on 2G isn't much different from not having the content at all. While 4K is rarely required, pushing out coding tutorials at any resolution below 1080p is nearly worthless, and pushing it out at 480p is worthless (Unless it's a retro C64 BASIC tutorial in which case rock on).

So you'd much rather watch absolute shit for 30minutes?

Yeah that's a hard no.

I agree with you - partly. It's either "watch nothing" or "watch not in 4K".

Not everybody in this world has such a good internet connection. There are still enough people that would be happy to even have internet...

Does Youtube work over 2G?

Work... Yes, you can watch a video fluently, but it takes (on Desktop, Firefox, with Throttling on "Good 2G") maybe ~2min to load the video page. However, you can start watching the video before the full page is even loaded.