Technically speaking, that's piracy. Legal websites can be used to do illegal things. There's no store legally selling the Windows version right now (except for thrift stores or individuals on Amazon/eBay selling their own out-of-print physical copies), so it's considered to be a kind of abandonware. That doesn't make it less illegal, it just makes it a little more morally acceptable to pass it around because currently there's no easy way of getting it.
The correct solution is to make the Windows version available on GOG. Getting the Windows version to work on newer versions of Windows is actually fairly easy. This page (https://pcgamingwiki.com/wiki/SimCity_2000
) will tell you all about running the game in general. I've tested this with my personal copy, it works well.
The Windows version has a variety of quality of life improvements, including a faster speed setting, a further away zoom setting, the ability to use higher resolutions, and better cheats (which are very important in a game like SimCity), among others.
However, the DOS version is still nice to have for its unique UI. Wizardry VII is available in both its DOS and Windows flavors, so I don't see why SimCity 2000 can't be.