Y only has to make the patches available to Y's customers, and GPL is satisfied/Y is not violating GPL and Y's customers are simply wrong in their argument. (I suspect that you know that.)
The implied "problem", that Y's customers are now essentially running on a private fork, not on the maintainers mainline, is no cause to force the maintainer to incorporate the changes. Forking is one of the freedoms in free software.
The implied "problem", that Y's customers are now essentially running on a private fork, not on the maintainers mainline, is no cause to force the maintainer to incorporate the changes. Forking is one of the freedoms in free software.