If you don't trust the other end then disappearing messages should be used, simple.

This is one of the problems with Signal having a bit of confusion about what exactly it's use-case is. There are plenty of cases where locking down the ability to save/view/export messages are valuable, and Signal provides tools to be able to do that. Making that the mandatory case though means that it's harder to adopt as a general-purpose communication platform.

The need to decide if the goal is still to get as many people off of SMS/facebook-messenger as possible, or if the goal is to provide extreme security to dissidents and protestors, or if they're going to spend the effort to be able to do both effectively and let you choose which conversations or messages get which level of protection.

But I could just screenshot all of it, and you'd never know. There's a setting to prevent screenshots, but no way to tell if other persons have it enabled or when their setting change.

It's like when Signal asks you to put in your PIN, but it's the same PIN you use to unlock your phone. There's a different Signal PIN, but that functions as a check for when you install Signal on a new device. Your regular PIN is just a repeat of your phone PIN, and thus adds exactly zero security.