Name one similar service even close to steam in users?
That's why they went with steam.
Some games are on Steam AND other services. It's a little weird that they make it Steam exclusive. I usually prefer Gamersgate and Impulse because I actually can keep the install files and such, I buy a lot of games on Steam because of really good deals that show up. I have a lot of modern games on Steam, over 40 I think, I only paid full price for around 3 or 4 of them... Usually 5-19.99$ is the price range I go for... For games that aren't even one year old usually.
My thinking is that if Steam indeed screws me over 10 years down the road, I probably won't be playing these games anyway, and those that I will want to play, I will probably be able to afford like 2$ and buy them on another service. As I just did for Civ 3 complete... 2$ on Gamersgate.