To quote Clem, head of Linux Mint: "At a time where GNOME applications are less and less designed to work anywhere else than in GNOME, a project like XApp is extremely important."
Libaidwata breaks backward compatibility with older gnome versions and amongst other things doesn't allow theming natively, so the Cinnamon team are going to have to fork off and maintain the older code which works so they can continue to have theming and stuff with Gnome apps.
Gnome seem to like doing the opposite of the Linux philosophy which says interoperability should always be a priority so that the code can be shared as freely as possible.
I can't tell whether they are stupid or lazy over at Gnome. It's not enough to strip the DE down to nothing but now even the code that worked with previous, gnome still widely used, is being dumped.