This is how it already is for MATE as long as they intend to keep it that way. It uses GTK3, which is currently in maintenance mode, and will only receive bug-fixes and nothing else.
Time that could have been spent improving MATE instead.
Because "bug-fixes and nothing else" isn't enough as i described in the comment you replied to right now as well as in a more detailed comment i replied to the parent post.
> If they wanted new features then it was not a waste of time. There are plenty of new features in GTK3 and GTK4, those are the main reason to port.
It was a waste of time because new features can be added in backwards compatible and opt-in ways that keep all existing applications and source code working while still using the latest version of the libraries.
Gtk4 breaking backwards compatibility with Gtk3 wasn't an unavoidable act of nature, it was a decision made by the Gtk developers, my comments are a criticism of this decision and its consequences for the developers that rely on Gtk.