For example, if WINE or something similar reached the point of enabling enough compatibility to allow many Windows games to play seamlessly, that might make a big difference.
That IS the difference. I actually like Windows(even 8/8.1) for the most part. I also like the idea of running Linux instead but my Steam library has over 200 games. I think about 15-20 of those titles work natively under Linux. Valve has to invest in WINE and perhaps hire some of the developers directly like they did for SDL to get it to a level where most of our games can work under Linux just as they do under Windows. There's no way I'm switching from something that just works for all of my productivity and gaming needs to something else that cannot run the programs I run.
Ideally I'd like to see them make their own distro with all of the drivers and WINE shit needed to just allow all software in my library to run just like it does under Windows. I double-click the title, it downloads and installs, and then I run the thing. Not all Windows games need to run perfectly and some might not ever run at all but that needs to be the exception and not the rule.
I'm not going to dual-boot. I'm not going fuck with a separate Windows instance of Steam through WINE which I have to configure arcane settings for each game I have. Make WINE good enough and integrate it with Steam so almost every game I "own" can work right out of the box or there's not even a choice to made about what OS I'm going to run.