Both ReactOS and Wine are open sourced, Microsoft engineers and lawyers look at their code. That's not a hypothetical situation, it happened in the past, the most recent news event on this was: https://www.theregister.com/2019/07/03/reactos_windows_resea....
With the risk of copyright infringement you want to be sure that you're taking active measures to defend your position in case you're facing legal charges.
Yeah, I actually once worked at a place that was clean-rooming. They went to absurd lengths, even WITHOUT the old source code.
If you were designing the new app you couldn't even -look- at the existing UI; you could only get descriptions. Every term was checked by legal to make sure that it was a 'standard' industry term vs something that was specific to the existing software. They even used a different tech stack wherever they could.
I think the other company still tried to sue, IDK how it played out (I was gone by then) but I remember at least thinking they were in a pretty safe spot with how they handled it.
With the risk of copyright infringement you want to be sure that you're taking active measures to defend your position in case you're facing legal charges.