> Some later Trek shows (e.g. Voyager) turned captains into geniuses who knew more about everything than all of their underlings.

Not everything. Picard was command track his whole career, I think the same was true for Sisko. But Janeway started as a science officer before switching to command, and Archer was a test pilot. Their experience in those fields would sometimes be useful. It's similar to how Picard's interest in archeology was sometimes useful.