Milsim in Ontario anyway is just a term used to describe a persistent game that doesn't end in rounds like a skirmish. Everything else is opinion. Airsoft games are a spectrum from plinking to pure milsim, not a black and white option.
True milsim, taking the phrase "Military Simulation" to the logical extreme would require replicating the conditions of real military operations as best possible considering the constraints of the capabilities of airsoft guns. Most players want a middle ground between fun and realism, because pure Milsim is more like a reenactment. The conditions would have to be very controlled and would, frankly, not be much fun.
You have to be careful as to what you claim to be realistic however.. keep in mind airsoft games are usually in the neighborhood of 40-200 players. Thus making them platoon/company level operations. Most scenarios milsims create far exceed the scope of these operations.
"Hokey religions and ancient weapons are no match for a good blaster at your side" - Han Solo
Commanding in Airsoft