1) People looking to play retro games
2) People looking to work with legacy hardware, especially in manufacturing and healthcare
3) People who want the comfort/familiarity of an older operating system
I'm always careful to issue a disclaimer that Windows XP should never be used for anything where you need security, in practice, I don't see much of an issue. The reality is that although XP is a tempting target in terms of vulnerability, it's not widely used enough to be useful to modern malware.
The machines I sell come with Windows XP Delta Edition[1], which as far as I know comes with all the available updates for XP already installed - no Legacy Update necessary. I've been using the Mypal browser [2], but will definitely try Supermium!
[1] https://xpdelta.weebly.com/xp.html [2] https://github.com/Feodor2/Mypal68/releases