APT of 15 years ago is a pretty far cry from APT of today. In fact APT was abandoned by it's original maintainer and kinda sucked until something like 2007 or 2008 when someone else started working on it again. But you know rose colored lenses.

WU has never really been about software installation or upgrade, it's focus has always been about bug fixes and security patches. Silverlight and IE are the only apps that have ever been upgradable via WU and for IE that's been a relatively new occurrence primarily driven by getting users off of older insecure versions. So really Silverlight is only app ever distributed via WU.

You also have to consider that APT was a solution to the horrible experience of getting and installing Debian libraries and applications which still is a pain in the ass without APT. Windows never had the problem, apps that were available for download were easy to download and install.

Keep in mind that APT is for opensource software and doesn't have to deal with authentication and authorization of software licenses.

And since WU isn't a store front and most Windows software isn't free, opening up WU to 3rd party apps would probably have come with distribution costs that I would wager most app developers wouldn't pay.

So again not a very fair comparison.