Nothing happens to them. They can't be jailed for taking their PTO or sick leave, they are free to quit and there are not enough recruits in the pipeline to backfill them en masse. They could be put on a PIP, or fired for not showing up, or some other retaliatory and childish Trump action, but what is the government realistically going to do to fix this? Fire them for poor performance and make sure that a short term shortage becomes permanent? How do you even recruit backfills when the entire world knows you fucked over the previous crew and promise to do it to the next batch too?
On the other hand, if privatizing ATC is one of your goals, this is probably an effective way to force the issue.
(Current Airline Pilot here, definitely NOT in favor of privatizing ATC, but historically breaking things on purpose is the usual path politicians take to privatize)
