It's the Budgie project that wants to move away from GTK, not the whole of Solus. Now that Budgie is a completely separate project the GTK-removal has even less to do with the OS itself, and will most likely only affect the Budgie edition. There will, for the foreseeable future, always been a e.g. Gnome edition, which of course means including everything GTK.
We can also add that
- MATE also relies on GTK
- Tons of applications in the repo are GTK-based
- Budgie won't move away from GTK in a few months
- ...
Efforts for running away from systemd would be just wasting time to replace something that works instead of using it to do something more useful, especially on a small project.