Their user base is going to want win7 support for sure, but the bigger question is DirectX support. DirectX 12 is the current version and microsoft recently announced that due to pressure from game manufacturers they were making it available on windows 7, starting with World of Warcraft. Reading through some links it looks likely it will become available for other games on win7 too. Since there is no DirectX 13, you have to ask yourself how far into the future before there are games that are ONLY compatible with DirectX 13, a product that is not even yet announced? Given that Fortnight just now announced the end of support for DirectX 10, I'm thinking there is a long future of support for games on win7.
Microsoft ports DirectX 12 to Windows 7
Fortnight ends DirectX 10 support