Based on a discussion in the December 2021 timetables thread, I think restoring all off peak services to pre Covid levels as soon as staffing issues allow should be a priority. Certain routes such as London to Birmingham, London to Chester / North Wales and some services around Sheffield and the East Midlands look like they will still significantly reduced from pre Covid levels until at least May 2022.
Significantly reduced frequencies is not going to help passenger numbers recover. If there is a train is every 30 minutes, missing a train by a minute may be just a nuisance, however if a train is every 60 minutes and that train then has standing room only, the passenger may decide it is better to drive next time.
Given the rail industry has very likely lost a considerable proportion of its commuter traffic, possibly for good, I think they need to be careful they do not drive leisure passengers away too due to significantly pared down frequencies and slowed down services from seemingly never ending ‘temporary’ timetables. If leisure demand is still subdued in two years perhaps then they look at reducing frequencies, but not now.