Frankly it's absurd to psychoanalyze Steam, saying that it wants to do what other companies are doing, and not to recognize that if it wanted to do those things it could easily do so.
From the Steamworks FAQ
>8. Do you require exclusivity for titles you sell on Steam?
>We think you should get your game in front of as many people as you can, therefore we do not require exclusivity on titles.
It's a monopoly in the sense that it's practically financial suicide for a games developer to NOT sell their game through Steam. Whether or not they really "have to" is besides the point if the reality is they'll not make a positive return on their game if they skip out on selling on Steam.
A few big publishers like EA avoid this by creating their own alternative stores and just hoping gamers will deal with it and install Origin, but most games studios have no choice but to accept that they're going to lose 30% because the reality is gamers expect to be able to buy any game on Steam.
This is just from observation without statistics, but I feel once people have a decent number of games on Steam (e.g. from lots of cheap purchases from sales) and no other store, they'd rather stick with Steam instead of installing another store app. Even if another store was a little better, the idea of having to install another app and have games split between two interfaces is a big deterrent.
Almost all Valve titles are Steam exclusive.