Valve has a right to charge whatever they want for their product, just as you their consumer have the right to balk at it and walk away if you think they've asked too much. Where exactly do you get the idea that you have a right to afford Valve's games?
I run a business myself and I charge what I think my products and services are worth. Should you complain that you can only purchase those from me and not from Wal Mart or Best Buy? And even if you could purchase from those places, wouldn't I just charge THEM what I'd normally charge you directly, forcing them to have to price higher in order to make a profit on what they'd already spent buying from me?
Your monopoly argument is invalid because you're pretending that Valve is an industry unto themselves, when they're merely a single developer in the larger gaming industry. Valve can't seize Doom 3 or BF2, put them on Steam and say that you can only buy them there. If id or DICE chose to give Valve the exclusive right to sell those games on Steam (which would never happen anyway), you are perfectly capable, along with other gaming consumers to tell them that you don't like this practice by not buying their games. If as you say, Steam is so anti-consumer, consumers will communicate that by not purchasing the games, causing Valve to abandon the system in favor of one that they can make money using. The system self-regulates.
Your wants as a consumer of videogames don't trump the creator's right to sell their products at the price and in the manner they see fit.
Interested to see your response...