The link is clearly not good out of Google ecosystem. You need to have a Google account to view it. Otherwise, there's nothing stopping the e-mail client (or even recipient's email server) from fetching the link upon receipt.

To be clear: Gmail want to be a UI for sending messages via a proprietary protocol which look like emails to the sender and receiver if they are Gmail users.

If your outgoing emails were silently converted into Snapchat messages (more features! why not! everyone's one Snapchat anyway!), would you be similarly unconcerned?

>and is clearly a gmail feature, not an email feature. There's no problem here.

That's precisely the problem: turning Email into Gmail.

Remember how IE added features to HTML which were IE features, not HTML features? Remember how it wasn't at all a problem?