You're probably right about Einstein not suggesting there was a hard physical limit preventing nuclear energy. But I don't think that hard physical limits are a

You're probably right about Einstein not suggesting there was a hard physical limit preventing nuclear energy. But I don't think that hard physical limits are a good argument anyway. There are FTL proposals that do not violate known physics, such as distorting spacetime using some perhaps yet unknown mechanism. Saying that there are hard limits in physics here, as you did, suggests that research in this direction is not worth our time. I believe that is misleading.