FTL is possible according to general relativity (wormholes). Getting negative mass or not getting fried by radiation are secondary concerns.Agreed. Also, at some point, all the science elite was sure that we can't go faster than the speed of wind. Or later, faster than the speed of sound. Or that it would be impossible to send humans to the moon. Or that we know all of physics, and the only thing left is more accurate measurements. Assuming that we know for sure that FTL is impossible, is just woefully arrogant IMO.
This is C2C canon too - at first there are gravity manipulation techs.
Then again there are time travelling techs, if you look at their descriptions you can see that one tech was given by your future civilization.
Maybe quantum general relativity will be more lenient - theory that would fix compatibility issues between general relativity and quantum mechanics.