Its design intended it to be used that way, but that's not how people use it. Everyone uses a centralized email provider now (Google, Microsoft, Apple, etc.), and these providers act as gatekeepers to email. If they disappear, people will find an alternative centralized messaging scheme, not start learning how to run their own mail servers...
- Even if it were true that "everyone uses Google/Microsoft/Apple/etc", the ones that don't can still send/receive emails from everyone else.
- If one of the of the big providers disappear, the system as a whole will continue to function.
If all centralized providers disappeared and I had to send a message to my friend, I wouldn't setup my own SMTP server... I'd just send him an iMessage. Or I'd post to a Slack group. The system without centralized providers is useless to most people using it today because no one wants to learn how to setup a mail server when there are dozens of alternative messaging tools.
As for the few that do have the capacity to setup a mail server, they would only be able to message others that also have the capacity to setup a mail server (again we're assuming no centralized providers and thus no delegation of this task to a trusted party). So maybe the system would still work for this niche group... but then we get into the bigger question: are there entities today that control the Internet and do they have the capacity to shut down systems like email? Tor?
I am pretty sure if Google disappeared we were going to see a huge drop in the email traffic. How would I get to know the new email address of my friend who we only talked using Gmail for example?
> Everyone uses a centralized email provider now (Google, Microsoft, Apple, etc.), ...
No, it's not the case that "everyone" does that. I don't, and many of my colleagues don't. Perhaps muggles in general don't, and perhaps you're really only talking about such non-technical people, but your hyperbole undermines your argument.
And I'm getting really tired of those big services automatically classing as spam anything that comes from a different domain.
> No, it's not the case that "everyone" does that. I don't, and many of my colleagues don't.
To add a little more nuance to that, millions (or billions?) of people working in various areas have email addresses provided by their employer with specific domain name suffixes. Even if those are using cloud based email providers like GSuite or Outlook 365, as far as someone who wants to correspond by email with these people is concerned (personal or work use), the email address is certainly not an @gmail.com or @outlook.com address. And all these people understand that quite well, even if they don't work in the tech sector.
> And I'm getting really tired of those big services automatically classing as spam anything that comes from a different domain.
This really bugs me A LOT! It's as if the systems have a knee jerk reaction based on domain names while they themselves wouldn't consider all their users (on their domains) as spammers.
It’s clearly decentralized as you cited multiple providers in your own post. These providers share no common (logical) servers. Would Apple go totally out of business every people that didn’t relied on this provider would be totally unaffected. If you own your domain and only delegate to a provider it’s almost seamless to switch to another. (Admittedly it’s more frequent for businesses than for individuals).
It's true that when or if you need to communicate with them, you need to play by their rules, but it's also true that email has the potential, the power, and the technical solutions to shift away from it, should it ever be needed or wanted. This is not true for centralised by design systems.
EDIT: I don't think people will be able to abandon email, given it's a replacement for paper mail (banking, official papers, etc). If the big players won't play nice, people will need to get to another provider, simply because the source of email won't be willing/fast enough to jump on [system X].