In general, yes, but there's a good chance more niche stores like Steam will survive for a fairly long while.
* Compared to e.g. either of the Apple app stores, Steam is a very open platform: there are very few demands imposed on the publisher, and developers can release updates whenever they wish, for example. The only other platform where the sort of iterative development Steam can provide is possible is the Web.
* It provides a tremendous value-add: infinite game downloads, automated patching and save file syncing in the cloud. Of course, none of these are issues with a Web-based game, but at least some of the stuff available on Steam isn't going to be possible in the browser any time soon.
* Games are generally much less fungible than the average application, Valve's own games even more so.
* Valve's earned the trust of most PC gamers, including mine. It's probably (percentage wise) the most trusted app store vendor currently in business.