Perhaps. Perhaps not. Considering there are dozens of other chat apps with huge market shares on both platforms, is it inconceivable that they had a better product than most chat apps? (Until quite recently iMessage was the only widely distributed and usable end to end encrypted messaging platform out there, for example.) It is Google who controls the majority of smartphones, not Apple, yet they have not been able to effectively "abuse" their market position, mind you.
Personally, whenever I carry an Android, I'd very much prefer Signal or WhatsApp than Google Messages app.
Bundling and control of defaults are powerful levers. If users were presented a ballot on first use then perhaps these incumbents would have to compete on an even playing field.
Personally, whenever I carry an Android, I'd very much prefer Signal or WhatsApp than Google Messages app.