While it doesn't seem likely at this moment, a nightmare-mode worst case scenario would be that Tor becomes compromised to the point that it would not longer be reliable. People in countries like China or Venezuela, or other oppressed countries would have a much, much harder time speaking against their governments or communicating safely with the outside world. In countries like the US, it would also become more dangerous to speak freely, because it would be much more difficult to communicate without the government collecting metadata on everything you did.