I would not consider this to be "living for ever"

Maybe not. But let's suppose there was no "real world" at all, only a huge number of Boltzmann brains, some of which, from a subjective point of view, look like continuations of each other. If for every brain state there is a new spontaneously appearing and disappearing brain somewhere that feels like the "next state", wouldn't this give a subjective feeling of immortality, and wouldn't it be impossible for us to tell the difference between this situation and the "real world"?

In fact, I think our current theories of physics suggest this to be the case, but since it leads to the Boltzmann brain paradox, maybe it actually demonstrates a major flaw instead. I suppose similar problems apply to some other hypothetical situations, like nested simulations.