Email is an open protocol. Anyone can write a client. All major ones work with Gmail. For YouTube, see newpipe. The main missing piece is Google maps (at least the navigation bit) which you can't legally get without explicit cooperation from Google.
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.
I don't know people here noticed but they are already doing so by bringing "confidential email", etc to all users which explicitly require WEB VERSION or Gmail app.