The problem of falling projectiles from celebratory gunfire is trivially solvable insofar as the solution to the problem is simply to not fire the guns. So in that regard, it is a trivial solution.

The practical problem is that despite risks, people choose to be stupid. I think even an uneducated person knows that a bullet fired into the air will eventually return to the ground.

The social reality is that humans oddly seem obsessed with doing things which are clearly detrimental to themselves or others, in the short or long term. This part of the problem is not at all trivial to solve (I doubt it can be solved).