Firstly, the prospects for this summer seem highly uncertain and overall quite pessimistic. There may be some improvement vs summer 2020, but perhaps not that much if travel restrictions are almost certain to continue in some form. Certainly 75% of summer 2019 seems unlikely. I think it is safe to assume there won't be any need for new controllers this year.
When considering longer term, I think it is more positive. The "end game" for Covid does now seem clear. By late summer, most major economies will have vaccinated large portions of their populations. With possible booster jabs in Autumn (and ongoing as needed) to counter new variants, it seems likely that Covid will be under control by the end of 2021. By 2022, most travel restrictions should be lifted. There will still be a longer-term, structural reduction in air travel (eg, zoom replacing business meetings, recessions), but this should be limited compared with all of the leisure travelers who will still want to go on holiday. 75%+ of summer 2019 in summer 2022 seems realistic.
How this translates to controller recruitment - who knows? There is a big backlog of trainees to be processed before they think about any new recruitment. then there are the trainees who were let-go last year who I believe have some sort of first refusal on any new vacancies.
In a scenario where NATS start requiring new trainees out of the college in mid/late 2022, those trainees would have to start college at the start of 2022. In that case, we could see recruitment in late 2021? This is very optimistic. More likely, I would say, is recruitment beginning again perhaps in mid 2022.
Disclaimer: this is all pure speculation
