hmm, thanks for your assumption that I do not understand how mail works.
tl;dr I am pretty sure I know
Mail itself works, you are right, the whole idea of mail is and always was a distributed system. But based on my assumption (which could be right or wrong, depending on your personal viewpoint), where a file server would just work a mail server would not work with the extra effort you were describing. And I am not saying the extra effort is installing just a mail system.
As you said right, you can do everything an make it work, but the actual problem with a NC integrated mail system would be, that people gonna wonder why their files a available but their mails are not being received on the other end.
The complexity and liability of running a mail environment is by far bigger than hosting NC, nowadays.
But also there, I hope that systems like mailcow, mail in a box, docker mail systems and all the other solutions to host a own mail server, do get used more often. This would bring mail back to its origins and not to what it is now (google and microsoft forcing the standards). I also hope that things like IPv6 would remove the IP Blacklisting databases and introduce new ways of identifying a trustworthy sender, or just push existing solutions to a broader use.
Anyhow, my solution was to move my mails to a local provider I trust for 14 years and get rid of my google MX, yours is to host it on your own. I hope, anytime soon, my mail system is also hosted locally, but setting up a working NC instance was/is by far easier - and a beginning to rebuild the Internet.
But I am also happy to discuss all of this with you on a private channel, because I am sure I could learn quite a lot from your setup.