I know a bunch of people (mostly gamers) who refuse to upgrade from Windows 7 to 8 or 10, mostly because of usability and privacy concerns.
Eventually, they'll have to (DirectX 12, but I'm sure there's a ton of APIs for non-games as well and drivers will stop being updated for Win7).
But if ReactOS can reach a level of maturity and compatibility to please such people, there's a sizeable niche there of people who:
1. won't switch away from Windows to Linux or BSD,
2. but would be willing to adopt a familiar and compatible operating system.