Don't get me wrong, I'd like to see viable competition to Steam (monopolies are not good for consumers in the long run) - but you're missing the whole point. Steam's position has never been stronger.
It doesn't matter to Valve that you can pick up old and low-budget games for cheaper elsewhere most of them time. There have been intermittent cases of GoG being cheaper than Steam on certain for a couple of years now. But it's irrelevant. Why?
First, these titles are a pretty small part of Steam's market. Steam is primarily about the higher end commercial market. Sure, the classic games are one of its income streams, but most people on there are either playing full-sized commercial games or monetised free to play titles like DoTA2. And in the former market in particular, Steam remains well ahead of the competition. Origin's pricing is better than it used to be on many titles, but it still struggles to match Steam on either variety or price point.
Second, Steam has a very, very aggressive and very potent flash sales model. Let's say Indie Game X is $9.99 on Steam and $8.99 on GoG or another competitor. Now, you could save a dollar by going for the GoG version. Or, if you're not desperate for the title, you could wait. Because come the summer sale, the Christmas sale or just one of the regular midweek, weekend or daily sales, you might be able to get that game off Steam for just $2.99.
Third, Steam is a lot more than just a storefront. It's also a fairly comprehensive suite of back-end functions, on a par with those offered by Xbox Live or the Playstation Network. Given a choice between a DRM-free version of a game and a Steam DRMed version, you'd expect most people to go for the former, right?
If you read articles like this
you can see pretty clearly that Steam copies of games are more sought-after than DRM free versions. People actually value the friends-list, messaging and other back-end services that go with Steam and they value them much more than they value concepts like the freedom to do what they want with software they've bought. That may be an unpopular sentiment on slashdot, but it is the way things are moving out there in the market.
Whether or not you agree with Steam's business practices, we are a long way from even starting to see signs of its decline.