Welcome to the forums.
I am sorry but your argument is not at all good. It's "valid" points are hardly valid on the surface, but not so if given any consideration. Everything you have just said has been said before and shot down many times, and shot down many more times than it has been said.
A little extra money you say? I think not. The whole thing is to make sure they get 360 sales. That's the gold. That's what microsoft wants. Sure they could release it for the PC also, but then the PC gamers could just buy the game, and not be swayed to finally buy a 360.
A PC gamer wants Halo Wars, they tell him hehas to get a 360. He waits for christmas, he gets it, microsoft makes 300 dollars + 60 dollars of the game, and 120 dollars off of the other 2 games he gets for Christmas.
Later on he buys 4 more games in total and buys some microsoft points.
They have just made 740 dollars off of Halo Wars instead of the 60 dollars they would have gotten normally if they made a PC version. You say well, not everyone can get a 360 for Christmas.
True, about 2 out of 3 can't get it. Has microsoft lost money for not making it for PC?
the three people that would have bought it for PC add up to $180. Compare that to the $740 they made off of the one person. So what, they discard the 2 out of 3, they loose $120, but they made $740 off of the 1 person. You tell me which is more profitable.
Now, you also seem to forget that it takes time and effort to port games, as they have to remake the game. The only way they could release sometime around the release of the 360 version would be if they started both around the same time, and they haven't, in fact they are not even planning on starting after they finish Halo Wars.
Alas, if you wait 4 years, you might get your beloved PC version. However, Halo plays MUCH better on console.