On one hand, you can probably still run your Windows software that you bought 20 years ago. That's the good.
That bad is that many things in Windows still work the same way that they did 20 years ago even if better solutions have been developed because they want to maintain the backwards compatibility. For example, programs are responsible for installing and uninstalling themselves. So if you have a program that doesn't ship with an uninstaller (or the uninstaller is broken) then there is basically no official way to remove it outside reinstalling Windows. There's no sandboxing and apps can do whatever they want to your system. You also have quirks like not being able to add more than 26 drives (because you run out of letters) and having to deal with multiple different settings apps.
I think if it weren't for its massive software catalog (which it has because of backwards compatibility) basically nobody would choose Windows over any of the less clunky operating systems out there. This is the reason that I attribute to the failure of every Windows version that Microsoft has tried to release that didn't have 100% backwards compatibility (Windows RT, Windows Mobile, Windows for ARM, Windows 10 S, Windows 10 X, etc).
So just to be clear, what I'm saying is that maintaining backwards compatibility at all costs will produce a clunky operating system that is not pleasant to use.