Not so much failed as just outdated. That's like saying NCP is a failed product. It served it purpose for a time and as things changed, it was replaced.
That being said, I don't know how SMTP hasn't been replaced yet. It shouldn't have lived much longer than the early 2000s, in my opinion.
its pretty telling the grandparent used the word 'product'. that's why we haven't replaced SMTP, because everyone is off building products. and they all want to own the whole world. SMTP isn't a product. its an open protocol standard.
(I certainly agree that we should have a new one, I just can't see how that would happen)
That being said, I don't know how SMTP hasn't been replaced yet. It shouldn't have lived much longer than the early 2000s, in my opinion.