This was my thought as well, but I still think we need evidence of this. If air marshals are primarily there to deal with terrorist threats (and not things like unruly/drunk/whatever passengers), then my feeling is that two things post-9/11 are responsible for the low risk of terrorist acts involving planes (or even just run-of-the-mill ransom-type hijackings):

1. Would-be hijackers can no longer get into cockpits. Pilots would much rather a hijacker kill every passenger and crew member on the plane than gain control of the plane.

2. Passengers don't take shit anymore. They know that, if terrorists successfully take control of a plane, the most likely outcome is that they're all dead. So they'll attack -- and hopefully subdue -- the hijackers.

I expect the effects of #2 have lessened somewhat, given that 9/11 was over 20 years ago, and the memory of it is less raw (not to mention many adults who fly now were young children or not even born in 2001).

If air marshals really do act as a deterrence, there must be some evidence to back that up.

(Nitpicking follows. I agree with your points.)

I don't think #2 actually goes away. It's a cultural shift. Prior to 9/11, the advice was always to just stay calm and let the authorities negotiate with the terrorist, because that had in the past generally led to passengers and plane surviving the incident. That level of passivity isn't coming back.