It does take CMs. One CM won't make a difference, but 5-10 at an attraction will. They will be pulling people from other attractions or not staffing certain locations.
Not being able to properly staff all positions can cause load/unload issues backing up attractions, making waits longer than they should be if they were running at peak efficiency.
Running half a queue and leaving the rest sitting empty never happens either.
I don't know how many times I've seen lines backed up to the entrance of an attraction because they have ZERO switchbacks open. The time in line would still be the same, but it wouldn't be backed up.
If half of the MK CMs happened to call in tomorrow, do you think the same # of people would be let in the gates?
How many consecutive quarters/years has Disney made a profit? For that amount of money, I haven't seen very many E ticket/high budget attractions open, especially when it comes to MK, the flagship park.