But I literally can email somebody from email-provider a to email-provider b. But, lets be realistic, if the email would be discovered/invented today, that would not be the case ;-)

This is true; every non-ephemeral messaging system on social networks is a replacement for email. A poor replacement.

If they are a poor replacement, why does anyone use them?

A better question is why does anyone still use email if these alternatives are superior.

Enter Google AMP for Email: https://amp.dev/about/email/

You can, though. Email is interoperable, which is why you don't see a bunch of people calling for gmail to be "broken up" (though there have been complaints about the weird anticompetitive stuff they've done to try and build a moat around gmail, I haven't seen anything come out of it)

If I couldn't send an email to a gmail account from a hotmail account then something is wrong.