Not only does FTL ignore physics established since 1910, it is lazy to pretend that it's the same only faster: the truth of relativistic physics is far more complex, interesting and just plain weirder than that. If you're aiming at "sci-fi" not a horse opera in space, why not embrace that?
Like my favorite novel accelerando is about the singularity, a concept many nerds were interested in about 20 years ago. These days we’ve all kind of accepted it’s a silly thing that doesn’t make sense but does that mean we should reclassify all of the fiction written around it as a concept as science fantasy? At one point tech has advanced to the point where they’re cloning humans so cheaply that politicians are cloning multiple versions of themselves to go campaigning. Does this impossible scenario make those books science fantasy, even though they get a lot of the science right elsewhere? Because if the answer is yes then we’d probably need to rename all sci-fi sections in bookstores to science fantasy because that’s the majority of what’s in them: fun shit with a veneer of science behind it.
It makes you feel a sense of wonder through magical abilities, places and creatures - it's fantasy.
It talks about advanced civilizations with unknown/impossible technology - it's sci-fi.
That's the distinction bookstores make, IMHO.