I'm not sure that is true. No one is saying external libraries cannot be loaded. It is actually a good idea to be able to rely on certain popular libraries already on the client; this makes a lot of sense. You can still have the existing mechanism so I don't think browser vendor are going to hold everyone hostage.

If you like, there could be mutually agreed upon standard repository that browser vendors routinely update from.

Sure, your less popular experimental library won't be in the list, that's what "<script src=..." is for.

It probably won't happen but it is hard to defend the position that it is a bad idea, I think.