Thats really easy for Reddit to measure. Why are you assuming they haven't?
It strikes me as a very shortsighted move.
If you stop assuming that Reddit is run by idiots, and you consider the likely probability that they've modelled this stuff in some depth, it's easy to believe that your initial assumption is wrong, and that the users are on 1st party apps (or will be if others shut down), and that many will stay and continue to post rather than leave or stop posting.
Your premise is based wholly on the belief that you know more about Reddit users than Reddit does. That seems dubious to me.