The people who are hung up on the plot are strange to me. It's science fiction with time travel, and the black hole mcguffin tying it all together is as good of a rationale as any.

If you want things that only have "hard" science fiction, there are other stories for that. If instead you grew up watching stuff like star trek and enjoyed it then maybe you can appreciate it for what it is: a story with a whole load of emotion and science fiction.

All time travel is bogus as far as we know. Stories with it can still be entertaining.