They don't expect. But usually some Gnome devs are GTK devs, so usually the whole things happens for a reason, and that reason is to have better APIs to be able to write better desktop applications.
GTK3 became "final" in 2016 with 3.22. That was when GTK4 development started. And many applications just started to be ported to 3. Then 3.24 as a maintenance release came out in 2018. And in almost exactly 4 years 4.0 is ready for general availability, but it's likely far from "final" and stable.
So GTK3 continues to be maintained for a good while. (Though GTK2 is now EOL.)
