They're cutting support for windows because Microsoft is, it isn't valve's responsibility to uphold a depreciated operating system. GOG is doing this because they are explicitly operated as a platform for supporting these old games, Valve is a general game marketplace without any real stated obligation to support older games if the developers won't.
Also this has very little to do with what you're talking about. It even states they are taking old games and supporting them for modern systems, not supporting old systems at all.
Edited to seem a little less confrontational
Yeah I'm not sure what they're actually doing here, GOG has always included fixes and patches for the games they distribute.