I don't buy the 'players will only give it 5 min and then give up' argument entirely. While it's true that some players will do that, that isn't exactly the audience that'll be shelling out 60-80 bucks for a detailed wargame without trying it first, either. The demo is free, so more people than its intended audience will try it out, that's a given. Those who give up within 5 min aren't necessarily lost sales, since they never would have bought the game anyway.
Demos are really for people who are interested in the game, but aren't sure if it's for them, and want to give it a test drive before making a relatively big investment. This is why you include tutorials and good documentation in your demos, so that players may learn the rules of the game and know what to expect from the full game. Again, players WILL do this; those who don't aren't going to will not be buying the game anyway, since it'd be even more complicated.
Airborne Assault is hands-down my favorite wargame series. I handily bought HTTR/COTA, and am there for BFTB day 1. The *only* reason I bought them was because of the RDOA demo available; allowed me to try out the system, learn what it was about, and realize how freaking awesome the gameplay was. I never would have spent 100$+ just based on AARs or word of mouth; I *had* to try it out. And I did, and I bought the series.
Same thing for the Decisive Battles series. Battles in Normandy had a good demo and tutorial, and I realized how neatly implemented the turn-based system was. Thus, once the Matrix Xmas sale was on, I proceeded to buy KP/ATD/BIN/BII and Battlefront all at once. Now, that WAS an impulse buy, but I never would have done it without a demo.
This is why I haven't gotten WITP yet, even during the Matrix sale. Sounds very cool, very tempting. However, that's a pretty big pricepoint for a game that might or might not click for me.
While I like demos, you yourself in your reply have given reasons why most small companies don't do them. Not only do you want the company to release a demo, you also suggest that company go to the time and effort of producing tutorials and documentation for it. Small companies don't have a lot of spare employee time in which to do that. That's why they have to choose which games to do them for. WITP is sort of an example of the opposite of that...with a game that enormous, how long of a demo is long enough to give the customer an idea of how the game works? How much information do you then need to give the demo player so that they understand how that very large, and very complex, game works?
Meditation on inevitable death should be performed daily.