It's about time. Windows has always been backwards compatible with the exception of off-system server requirements. You can still technically install and run old Steam builds, but sadly you can't run those old games now without upgrading your OS. Do an in-place upgrade to W10 at least and you keep your library. As the world progresses, I both love and hate to see backward compatibility. Apple is easily the worst offender here, closing out support for 2–3-year-old applications. For closed platforms, you can only expect so many years before you either have to let it go or sit with your library and enjoy what you have.
Windows 7 is 15 years old. I don't want application developers wasting their time on the 1% of people who can't leave a 15-year-old platform. NGL, I enjoy whipping up some DOSBox and getting all techy with things I grew up with in the early 90s. If you want new, you have to invest into new. If you don't, I'm glad you're enjoying your nostalgic experience, and you can sit comfortably with what you have! Unlike Apple, who forcibly blocks you from such experiences.
Any modern platform must move on from Win7. Win 8 isn't even worth mentioning. I support Steam moving forward and investing their resources into future development.