The suspicion with disappearing messages is that it may 'disappear' only from the front-end. If there's a warrant or a vulnerability I have no way of knowing if my 'disappeared' messages won't still be seen but with the 'hidden' flag set to 'true' instead of 'false'.
But WhatsApp currently uses End To End encryption. At maximum the only information left on the server would be the metadata of the message (which could still be relevant).
Is this verified? Didn’t the Signal founder quit working with WhatsApp because he didn’t believe it was actually encrypted (from Facebook)
Nope.
Jan Koum quit WhatsApp and stepped down from Facebook board after raising privacy and encryption concerns.
What keeps me from making a screenshot?
That’s not the point, nobody gives a shit about your screenshots. This is about preventing unintentional retention.