Cannot tell you for everybody , but for ATC we can keep partially current on simulators,most large facilities have their own , and the traffic will not restart at the flip of a switch , it will be gradual and people will be able to re- validate fairly easily as traffic comes up again ( if it does) .
The more immediate danger I see for us controllers is the actual number of controllers left when it will restart. If ANSPs are getting rid of staff , some are already planing a 50% lay off, there will be an issue as all previous experience shows the vast majority of them do not come back in the profession.
And recruiting and retraining new controllers take years. If we get over the peak in the next 4-6 months , if may work for us , if it last more than 2 years as some are predicting , this will be a major issue.