What one has to bear in mind is, that the customers create a demand, and ATC has to deliver the required service, no matter what the staffing level is. There's the constant pressure to minimize delays, to improve services (more direct routes), to keep costs low, to cope with the traffic increase, and still be able to deliver the services required.
So what happens first, if ATC agencies are understaffed?
Thinning out the roster, longer time on the sector without breaks (even beyond 2 hrs), resorting to single manned operations at certain times, going to the minimum with leave and days off. Assistants? Not required anymore, controllers can do this themselves (plus saves personnel costs). Planners? Coordinators? Scrap 'em (saves personnel costs as well). 7 hour single manned shifts in towers? Not a problem (and guess what: saves personnel costs again!).
And only when the staffing level has reached such a critical stage, that even none of the above mentioned cuts can ease it anymore, only then the customers are being affected (keeping sectors combined and tightening flow control with more delays resulting).