Spam is just one of many things that slightly reduces the reliability of email, and Bayesian filtering has reduced it to a minor nuisance. The implication that email was ever completely reliable is false, there has always been a risk of mail servers going down, for example, but it was also reduced to a minor nuisance by modern redundancy techniques (for example).

The implication that there even exists an actually reliable way of communication I don't think is true either, that's why email is still so widely used.

I think it's a trade off of having a decentralized system vs. a centralized system. Just imagine if we could only use Gmail or Facebook for email--now that would be just plain scary.