Popperians claim that we don't need any theory of uncertainty to explain how knowledge grows: uncertainty is irrelevant. That is an interesting claim don't you think? And if you care about the future of humanity, it is a claim that you should take seriously and try to understand.

Popper's views are out of date. I am somewhat curious about why anyone with access to the relevant information would fail to update their views - but that phenomenon is not that interesting. People fail to update all the time for a bunch of sociological reasons.

if I accept a bet from someone that a universal theory is true, would I ever have to pay out?

Check with the terms of the bet. Or...

Consider bets on when a bridge will fail. It might never fail - and if so, good for the bridge. However, if traders think it has a 50% chance of surviving to the end of the year, that tells you something. The market value of the bet gives us useful information about the expected lifespan of the bridge. It is just the same with scientific theories.