Email indeed is the perfect, and oldest, decentralised social network for communication. You do not need a particular platform to be part of it. But that is also what makes it terrible. If you can’t control the inlet, it has to flood the house - bring it to stand still.
That’s the one reason that completely negates all the positives for the e-mail as communications platform. So many smarter people have tried making this a usable system. Alas. No success till now. Getting a decentralised platform up and taking it mainstream with its innate complexities is a huge ask, especially amidst the pull of the simplicity of silos. Just look at mastodon.social.
Simplicity matters. I struggled for days explaining to my dad why he need not have a “www” in his e-mail address. And not have a “@” while typing in the webaddress for Gmail. After a point, he stopped using e-mail, nudging me every now and then on why I wasn’t on Facebook where he could just @ me. Apparently, “it’s a closed silo” is not a reason enough.