I think PEP 729 – Typing governance process | peps.python.org is what you’re looking for: the Typing Council is mandated to make the type system usable and useful.
However, I would caution against blanket statements that one concern should always trump another. “Implementation convenience” is important for making the type system useful, because a spec that is never implemented cannot help users. And that’s a real concern, as PEPs like PEP 646 are taking a long time to get implemented by all type checkers. I think it’s reasonable therefore to try to cut the scope of type system changes to ensure that they are implementable more easily and users can reap the benefits of the new feature sooner.
I don’t see a problem with the change @mikeshardmind cites above. The feature was found to be hard to implement and, importantly, there were no known use cases at the time, so it was cut. Now there are known use cases, so it would be good to revisit that decision. If anyone wants to re-add support for Union[*Ts], I’d encourage them to go through the process for updating the typing spec.