Retro computing is a fun and archeology. Preserving legacy software is a method to let us not to lose memories on technology history and evolutions. It would be a loss to watch let them die out in time.
And there do are actual usages when commercial maintenance for legacy systems is needed.
We well know that running unsupported legacy systems is dangerous, and will only do this on VMs or dedicated offline hardwares for pure non-commercial proposes.
That's also why the Internet Archive and the Computer History Museum (Mountain View, CA) are created.
MS (and many other software vendors) should preserve their support & download resources for legacy softwares, at least throw them onto dedicated archive sites or work with 3rd party organizations, instead of violently delete them from their servers and only make them accessible to internal staffers. Of course it's on their own discretion.