Because same thing happened with LiveScript, they associated themselves with something related(Java), and we all kmow what JS is today.
Similarliy, thrive could Thrive too, as when ppl search for " games made in Godot", thrive will be one, and thrive being such a big project, it is highly probably it will bring new GDscript Developers.
And godot will allow the thrive developers to focus more on development than of engine fix, if any engine problem comes, we can report it to the all dedicated GoDot developers.
If it’s about smooth transition, if we could get enough godot developers, they themseleves will help port cell stage to the rest game.
This early decision can make the difference for all the stages from years to months.