If steam decided to change it's client to only support Win 8.1 and 10 and blocked anything earlier, would you be having the same argument? They pick a standard and that's what's supported, the only difference is that MS have their own delivery method rather than you're typical EXE. This is not something specific to Microsoft, it's the same for every dev team out there. The more variables you need to support whether that be install method, Operating System or whatever else the harder the development work actually is. If a game has more than one installation method then you effectively have to QA your product twice because you have to be sure that the install works and those assets, the engine etc all function correctly after install. It's still irrelevant, it doesn't matter whether it's 10% or 90% of the development cycle, it still counts as time and effort. Microsoft is simply pushing the Store via their games, not the underlying game tech. Compared to engine/assets/network development, which for most publishers are OS version agnostic, patch and install engine have insignificant development cost.

Windows 7 gaming pc