If ReactOS were "ready" right now (i.e., could run 99% of all Windows 7 software unmodified), with all the bad press that Windows 8 gets, it might even have a chance to steal some market share. However, I don't think they can catch up before Microsoft manages to release another "good" version of Windows, and then the game of catch-up will start again.
I hope I'm wrong. I'd really like to see this succeed.