Your thoughts on Keychain (technically "iCloud Keychain, since "Keychain" is just the secure database that has been on Macs for more than 21 years) are a bit dated now, since iCloud Keychain (or "iCloud Passwords" as Apple calls them for Windows folk) is now available on Windows and in the Chrome browser.
Set up iCloud Passwords in iCloud for Windows so you can manage and autofill your passwords on your PC.
support.apple.com
So a solution exists for your Desktop for passwords, at least.
As for Apple Pay, that's intentional since Windows computers come from many vendors and it's impossible for Apple to control the overall security of the system. Could you imagine if Apple allowed Apple Pay to work on Windows without the same strict safeguards in place on iDevices and Macs and customers got hacked? You got it ... it would be Apple's fault.
Apple Pay has only been approved for use on devices that Apple can prove keep the transaction exceptionally secure. They can't provide that assurance across a wide arrange of Windows computers, otherwise you'd have your beloved Apple Pay on Windows.