In sorting through a bunch of roll calls ships recently are sailing anywhere from 50 to 90% full. Crew staffing if obviously slightly short, but what I think the OP is referring to are a couple situations where a large number of staff from various areas came down with covid. In one case it was many theatre performers. In other several wait staff.
Really simple if you lose 10 waiters then service in the MDR will be very slow. The issue is impossible to predict ahead of time just like the cancellation of thousands of airline flights.
With processing of crew visas taking weeks instead of months and the number of covid cases on the rose again; one needs to be flexible as none of us knows exactly what will happen on board. Same way you could test positive and be denied boarding or be quarantined on board.
If my parents were in their 80s, unless they could drive to and from the ship; I would not have them on a cruise now.