These two kind of go together. I've written about this subject before back in 2009: http://zerias.blogspot.com/2009/05/how- ... e-can.html
Okay, yes, that post is a bit on the dreamy side in aspect of micro-payment potential, but the rest of I think is still very pertinent. Commercial publishers, and for that matter commercial developers, don't know how to approach Linux for multiple reasons. How do game publishers handle the packaging question? How do game publishers handle the API questions? How do publishers handle secure purchases?
In fairness some of the issues have been addressed in the intervening years. Android/Linux has helped force the issue on API's used by developers. The Consumer Desktop Linux market is largely split between Distributions on Debian(pure) or Debian(Ubuntu), to the point that if you target Debian(pure) for development you can probably be pretty sure your application will be compatible with whatever /Linux the downstream user has installed. Application stores such as Steam, Itunes, Amazon Android App Store, and GooglePlay have gotten the average customers used to the concepts behind central-package-management and package-repositories.
I think there is a lot of room for Valve to move within Desktop /Linux in respect to games. However, that does come with some caveats. GNU/Linux is not going to be an automatic million-dollar maker out of the door. The commercial games industry has shot itself in the ass so many times on the /Linux subject that most of the commercial games target market no longer cares. Granted the inability of the commercial games market to target Linux has been a fueling factor to the explosion of Independent developers: case in point being things like Humble Bundle, Indie Royale, and Kickstarter.
I suspect that a lot of the Desktop/Linux market is going to be looking for parity first: e.g. games they already have on Steam becoming available for download and play on /Linux. But Valve's been in this position before when Steam was just getting started on Windows, and when Steam first hit OSX. I'm pretty sure that Valve has a handle on what it means to grow a market, and customers will always be looking to purchase entertainment.
As far as non-game in relation to applications I'm not sure there's a lot of immediate room to work. Valve has hinted at a desire in the past to get involved with pushing commercial software packages through Steam and not just games. This is probably a smart move on Valve's part.
However, there is no shortage of non-game software within GNU/Linux, a factor of the commercial market's inability to get consumer software products onto GNU/Linux. Some of the major applications consumers use today, such as Firefox, Chrome Browser, Libre Office, and VLC, have roots in /Linux development. Most GNU/Linux distributions have a wide-range of software preloaded to address most common computer usage requirements, and pretty much all GNU/Linux distributions leverage package-repositories for additional software packages. E.G. major point of using a GNU/Linux to begin with.
Where I think there is a lot of future room to work is within the definition of non-game but still pertaining to entertainment. Expanding Steam to include movies, music, or even e-books, would help broaden the appeal of the Steam platform.
Another long-term factor would be Steam on Android in general. One of the benefits to the Android software ecosystem has been the competing application stores and the ability to load applications that don't come from stores. Sony's gearing up to enter the competition with Playstation Suite, which will connect with other Sony services for music and movies. Sony seems to be setting themselves up where they sell content without regard to hardware. Buy a movie on Playstation Suite and watch it on your PS3, PS4, Vita, or any device capable of running Playstation Suite.
What I don't know is whether or not Valve is actually working with Sony on this positioning. I can very easily see a deal going down between Valve and Sony for SteamPlay Content to automatically include access to that content on Playstation Suite, and vice versa.