Active Desktop was always optional, wasn't it? And was it really used for advertising by default? I thought it was mostly arbitrary browser windows that you could attach to the desktop.
It's both. Having ads on some apps doesn't make it not an OS anymore. I won't deny it's an advertising platform, just like android or ios or television or radio or the internet. And while it's equally annoying on all those media/systems, it doesn't lessen their or usefulness
There are ads in the Windows UI. I've been an iOS user for 9 years and can't recall seeing an ad in the OS. So it does not seem "just like" iOS, at all.
https://en.wikipedia.org/wiki/Active_Desktop