It used to be possible for apps to just say "I'm gonna be the default for this, kthxbye" on Windows in the past. It did not go well in practice. Most users aren't going to be able to muck around ACLs on some config file; they'll just have their defaults silently hijacked, in the worst case by outright malware.
The sensible approach here is 1) a stable API to allow the app to request this, 2) a sane UX provided by the OS that makes it clear to the user what's going on and requires their explicit and definite consent, and 3) a legal crackdown on all abuse of market-dominant position - such as those injected Edge banners.
The sensible approach here is 1) a stable API to allow the app to request this, 2) a sane UX provided by the OS that makes it clear to the user what's going on and requires their explicit and definite consent, and 3) a legal crackdown on all abuse of market-dominant position - such as those injected Edge banners.