Yes, the protocol seems dated, but it is well-understood and implemented by thousands of vendors. It's a miracle that we were able to agree on a protocol, and on top of that, agree on a protocol that is relatively decentralized (although it does depend on DNS if you prefer not to use an IP to email). It isn't like Twitter or FB where a single vendor controls the platform.
It wasn't that miraculous, actually. Remember that it was invented before HTTP even existed. It was developed at a time where the only contender was Usenet. Internet was not yet used for any commercial purposes; there was no concept of platform. Internet, and thus email, was the medium between universities that we know of. Sendmail was included by default in each UNIX from that moment on; when you connected to the internet, you could already send emails.
The conditions were very different, it doesn't make a lot of sense to compare it to what we have today.