The replacement API that GMail offers (which reflects their different approach) is open and free to use, so it's not really an issue, as third-party apps can support it. And many, in fact, do.
That's not the case for YouTube, though, which is exactly why it's a problem when GMail isn't.
The Gmail concepts do not really map very well to IMAP and traditional mail clients. They do provide a custom API that does mirror their semantic for that.
