Gabe Newell certainly disagree on your statement of MS' trustworthiness. For Nintendo, they themselves have helped MS out by accepting the CoD 10 year deal. But that said I totally understand and respect your opinion even if I don't share it.
Gabe is in a different position though. Steam has been doing very well even without Call of Duty, so clearly there's not that much threat against Steam even if Call of Duty strayed away from Steam again. Additionally, Microsoft has demonstrated its willingness to put its games on Steam for years now, probably because that's where the money's at on PC. That is, Gabe has good reason to believe that Microsoft is actually interested in releasing its games on Steam, and even if they weren't, Steam would still do just fine. He can definitely afford to be trusting. In the console market however, the belief seems to be that exclusives matter a lot, and I'm sure both Microsoft and Sony are looking further than just the next ten years.