Software similarity and market positioning don't really come into consideration once the role of gatekeeper has been established.
Also, it costs the vendor to implement support for installing other software - resources the vendor could have spent on features I value, rather than features I don’t want. If only a government didn’t dictate to the vendor what it should do, stripping the vendor and the user of the power to decide for themselves.
As an anecdote myself, the main reason I haven't switched to a Galaxy S24 is because my Airpods work amazingly with my iPhone and Macbook, and my Apple Watch only works with iPhones. But very often I sorely miss having Termux, NewPipe, Tachiyomi, a non-gimped version of GBoard, Syncthing, a sensible launcher, and probably other things that I can't remember off the top of my head. I've decided that I value the Apple system more than the value I get from those apps but this regulation means I get to have my cake and eat it too.