That works the other way too...people who invest during a bubble are quick to say "it's not a bubble!" - I'm not saying this is a bubble (I won't pretend to know enough or have clairvoyance), but it's not a valid defense to say something isn't overvalued just because people had said that at prices it has since eclipsed.
Indeed. Saying that it's too late or that it's just a bubble means nothing, because the truth is that it could be either of those things, and no one will know which until everyone knows.
So said people at $3, $30, $100, etc.