There's this common but, in my view, fallacious notion that "big ideas" science fiction must be the sort of hard science fiction that speculative physicists write: basically a story framing of some ideas they're playing around with, and thus is meant to be scientifically plausible. In fact, the main purpose of such hard sf is to posit certain scientific possibilities. So some readers and viewers approach every such sf movie with that in mind, which I think isn't the way a film like Interstellar was intended. It is about big ideas, human and cosmic. I don't think the intention was to make the science perfect, but just good enough to carry the larger payload of the story.
Or to put it another way, big ideas science fiction and "pure" hard sf are not synonymous. They often crossover, but they can be two different beasts. Interstellar was made in the spirit of 2001. If you have problems with the science of Interstellar, then 2001 is a total mess. But neither film is primarily about the science - that's just the setting and context for the deeper ideas of the stories.