What doesn't work great about email? easy to use, easy to set up, easy to modify, easy to switch which software handles your email both server-side and client-side....
But people posting tonnes of GIFs, emojis, and tagging @all on #general isn't spam? I don't disagree that email has a spam problem (though spam filters have gotten quite good in the past 20-30 years), just that spam isn't an email problem.
It's trivial to ban someone from Slack. The problem with email is how there's no authentication for incoming mail, and it's trivial to spoof the sending address so you can't block them. Sure spam is a problem on most platforms, but unauthenticated ones have it orders of magnitude worse than authenticated ones.
Every submission system has a spam problem. How is email any different?
I'd actually argue that email is one of the better systems, as I can manage and control my whitelist/blacklist myself. It makes things much more functional that way.