Anything trying to be compatible with a large, greedy company's software is a huge risk if it starts cutting into their profits. Maybe nothing will happen but something might happen. I'm more concerned for companies like EnterpriseDB than FOSS projects like ReactOS, though. The lawyers do prioritize on those making money with the competing software.
I wouldn't be surprised if we open-sourced NT, at the rate things are going. Windows devs are likely flattered by ReactOS at this point.
> Even if they abandon the project it's open source and work can continue even after MS has their way with them.
Such infringement claims are damaging to the legitimacy of the codebase itself. Anyone taking up the project could face the same problems.