One user wants it not to be shared, but the other user might have various reasons to want to take that screenshot - maybe it's evidence of something they need to share urgently with others, whatever. It's definitely hostile to that other user. I get why you'd set it up that way, but it's a tension that really goes against the "full control of your own device" ideal a lot of people have.

Don't send me anything you don't want screenshotted. Easy-peasy. I'll accept an opt-out of whatever protections are in place for the general user, but I don't accept that those protections should remain in place for all users. It's hostile.

Honestly, even schoolkids know to have another phone/camera(usually a friends) take a picture of their phone screen.

In a world where people have multiple old phones lying around it isn't that hard to come up with this workaround.

If you send it, it is no longer yours to control.

If it is my phone, it should be mine to control. Too often it really isn't my phone...

It's mostly dumb because of obvious analog loop holes. I at minimum carry 4 devices with cameras, often as many as 12. If I want to capture the disappearing message... I will do it; making it annoying just makes me pissed at the developer + the is.