Your argument flips over half way through. Assuming people wanted Gtk3 to natively render in Wayland then the need for XWayland would be lessened by one less piece of software that was dependent on it.
And besides that the main reasons for using Gtk3 is that code is more portable, supports hardware acceleration better, there are more widgets and layouts, Unicode support is better, theming is better, widgets scale properly on high DPI displays and it supports gestures and multi-touch.
Maybe none of these matter to an audio workstation. Perhaps the effort of porting is substantial and complex because Gtk3 does have some breaking changes to its API. But I imagine that even this software would benefit from button texts and widgets scaling properly on 4K displays.