Windows 7 was great and I'd love to go back. If I really had my druthers, Windows 2000 was peak and XP was just a vulgarized version of 2000.

However, it is Microsoft more than anyone else that has decided to stop supporting those operating systems. Windows XP does not have support for any modern version of TLS (only TLS 1.0). There's no good way to support a browser-based app like Steam on a platform that cannot natively provide a secure connection to a modern web server.

There is not such a hard reason to drop Windows 7 support (again, except that Microsoft no longer supports it) but there are security-relevant APIs that are only available starting in Windows 10 which means special patches would have to be maintained just for Steam on Windows 7 to continue working securely.