Yeah. You can tell Email is old. very old. The internet has exploded since then. There are so many more nodes and users out there than anyone would have imagined in the early 80s. Also technology has advanced together with how we use it. 7-bit is madness by today's standards. Of course Antispam and E2EE hasn't been baked in because it wasn't a thing back then.
But things are different today. I don't think there will be another 'explosion' so that the requirements to such a protocol and it's usage will change as quickly and fundamentally.
Funny thing is, the resource usage of my mailserver or XMPP server is so much less than for example my Matrix server or any of the other 'modern' federated things i tried... And we should learn from XMPP's history. Both good and bad things. It's a complicated story and there is more to the story than just network effect or technical issues. And I love and hate Matrix. I'm glad it's there but i also wasted several hours looking for good client for linux that isn't element and uses all of my RAM. And fought with encryption in some python libraries. Sometimes matrix just isn't fun. Especially the encryption bit.
I don't care for the network effect. I have used both XMPP and Matrix. There was a time I could reach all my friends via XMPP. Back in the days when both Facebook and GMail had XMPP and WhatsApp wasn't there yet. As of today. I use Matrix. And the few people I talk to most frequently also use Matrix. And that's enough for me. I don't care if 99% of other people use something different. (Also there are bridges to other protocols). It's the same with Lemmy. I wouldn't be here if it was important to me to be on a platform with 1.5 billion other users.
I think we have an opportunity to do it right. And to design something that will last for quite a while. Of course there are issues to solve on several levels. Unfortunately back in the days, protocols were invented by scientists and to connect universities. Todays platforms are implemented by mega-corporations and their motive is to gather data and sell advertisement. So we probably need regulations and politics to force something like interoperability into existence. And of course there is the age-old question of reform vs. revolution. Iterative change sometimes isn't good enough. I'd consider email a case where we need revolution. I'd happily use some free sucessor. Even before the network effect or regulation kicks in.