I think many are simply not doing anything until they have to. Once their app quits working, they'll make a change at that time. Of course not everyone uses Mobile. Personally I don't use mobile much for internet access, mainly desktop browser. In that case I could keep using Reddit same as before, but I don't want to.
So the question is what portion of Reddit users are on mobile, how many will relent to the Reddit app, how many will quit using Reddit altogether, how many will look for an alternative, and finally how many will land on Lemmy or Kbin. Could be a lot, could be not many. We'll find out pretty soon here.
For those that do come this way they'll have a transition phase. There's a lot that's different here and takes some getting used to. Also the lack of certain features may result in some angst. There's bugs too. They fixed some stuff with 0.18.0, but searching for and subscribing to communities is giving me fits now.