It isn't as cut and dry as "not enough to shoot at, not enough action, failing is not fun" - all of those statements are not black and white, there is tons of grey area.
Finding the line in those is what we have to do. Failing a mission due to 100 AI swarming you at once can be very not fun, but failing due to being worn down through multiple engagements can be fun. Failing due to incorrect intel, the target escaping, enemy reinforcements push us off of an objective, etc - these are all fun ways to fail.
I think this might be more of the issue with people not wanting to fail - failure always seemed to be previous a lot of enemies just overwhelmed or shot us enough to make us get frustrated. While i won't discount that being a viable way to fail, when that's all you see, either mission complete or total wipe, it gets... old and stale. And that makes it not fun.
As for the "not enough to shoot at, not enough action" - I do agree with that at some level. We are here to do Milsim, but in the end we need to all realize that the actual part of milsim we want is the operations, stepping outside the wire, doing things. Something at some level has to happen otherwise its a waste of time, hence why we don't have FTXs or trainings where we all get together and clean our guns, do PT, etc.
With that in mind, action doesn't have to be "firing guns until we run out of ammo or targets." I'd say a mission where we sit at the airfield for 1-2 hours literally doing nothing is a failed mission, and is not respectful of the time the members put in to showing up. The good news is there is a middle ground, in my mind. Instead of sitting around doing nothing, send us out. Give us a mission where we have to recon and avoid enemy patrols, infill as stealthily as possible, slip in under the radar, go covert. If done correctly, we don't even need to engage many contacts at all, but its still what I would call "action."
Basically, things still need to occur during our missions. Events still need to happen. But that doesn't mean every mission needs multiple waves of endless enemies to shoot. Take out a small patrol with silence, slip past a defensive perimeter, do our shit and get out, with them none the wiser until daybreak and they realize their patrol is overdue. That would be fun for me, even if not full of "action" (unless we fuck up and get caught).
Quick note on intel gathering not being fun - Sitting on a hill for 2 hours looking through binoculars and doing nothing else sounds boring. However, gathering intel for a while and then using that intel to do something would be fine, and could work nicely.