I don't think the issue is actually the music player, dialer, home screen, messaging app etc, because Google have done a great job making everything like that r

I don't think the issue is actually the music player, dialer, home screen, messaging app etc, because Google have done a great job making everything like that replacable. The actual missing piece which is worrying if you want to go this way is Play Services - that provides loads of the APIs apps need (probably including some for those replacements) and is closed source.