It's the fault of copyright. Restricting what shows you can stream to your users instead of, for example, being required to pay a royalty, inevitably leads to this situation. Netflix being the sole company allowed to stream every show and film would result in a monopoly that would be bad for everyone as they progressively sought to increase profits year over year. One company having all that power would not be a good thing for anyone, including content holders.
The solution is simple: every streaming service should be allowed to stream every show/film in every country. Then, piracy can only compete on price. That requires significant copyright reform, however, and is very unlikely to happen.
GIMP's GTK3 port was finished several months ago. What remains to be done for GIMP 3.0 is bug-fixing and porting to the new Plug-in API.
The best way to upgrade to GTK4 is to upgrade to GTK3 first. There was some talk about working on GTK4 soon after GIMP 3.0 is out, but whether that will happen or not is uncertain.