Actually, it’s already useful for some folks. Sure, if you’re after a 100% feature parity to say XP, that’s not happening any time soon, but there are already folks using ReactOS in niche cases like embedded systems, especially in old systems like CNC machines, scientific instruments, industrial/factory machines and so on.
The main problem with old machines like that isn’t that they were running Windows NT4 or XP, but the fact that the hardware they’re using is breaking down and it’s getting increasingly difficult to replace it whilst maintaining compatibility. ReactOS is basically the only “supported” OS that still is compatible with those old specialised drivers and apps, while still being compatible with somewhat modern hardware.