What Gabe is doing is to try and break free of Microsoft's stranglehold on PC gaming. Things will be slow at first, but losing compatibility with older games is nothing new, it has been happening on PC for long enough and consoles have been doing it since the 80s.
Any old games I want to play are on Steam and Valve are shovelling a lot of money into porting them all over to Linux/MacOS. I will be taking the plunge into SteamOS when it is released and I doubt I'll go back to Windows if it does all I ask of, an ability to play my Steam Library and the ability to carry out basic computing tasks (browse web, YouTube, play music, etc.).