We send confidential docs regularly to users, who need access to those docs for perhaps 1 week at most. No one wants / needs to keep these around, but no one goes through their email carefully to delete these items.
If that users email was hacked -> they have a big problem. If we can mark the items for a 3 week retention and then expire those items for them, that great - and this lets us do that.
The whole I can wireshark my network -> 99.9% of the confidential info we send goes to other folks who ALSO want to keep it confidential. Getting rid of stuff you no longer need to maintain is a key way of helping avoid big document dumps.
The proof is in the pudding. Either this will help google sell to business (it will in our case in a big way). Or folks will say it is a stupid feature and decide idiots like Protonmail who can't seem to understand the point of these features now deserve our business. My confidence in a place like protonmail goes down based on this, and I'd love to get a feel for their security history and overpromises (ie, webmail client has got to easily be able to log and hack encryption etc).