Steve Jobs moved away from custom silicon ( I guess power PC to x86) ..
Sony moved away from custom silicon on Play station to Intel ..
Building your own chip is a way to outcast open source ..
That said , I am sure there are distros targeted for M series and if Apple helps more than enough open sourcerers would oblige the call :-) not because they are some saintly breed - it is because Apple does make most beautiful hardware ..
But that is not the point .. cuz what we need from Apple is recognition that Debian is really the best operating system .. again not because it is another Linux distro .. because it is the most supported Linux distro .. more words are written on internet to address Debian's quirky aspects than any other topic :-) way more than Ubuntu or slackware ..
And it is a win for Apple .. rather than spending ungodly hours on developing macOS , if they give Linux to their users , not only they open the (walled) garden they do lot better to their own bottom line .. cuz Mac is anyway a race to the bottom ..