^ But the thing about FC, just as with TWOK before it, was that you didn't need to see the tv episodes which preceded them to enjoy the movies. Maybe it helped. But really, all you needed to know was that Picard had a history with the Borg or that Khan had a history with Kirk.
No in-depth knowledge of either BOBW or Space Seed was necessary, any more than you need to know the ins and outs of the Clone Wars to enjoy A New Hope or to know all about Robert Baratheon's overthrow of King Aegon to enjoy A Game of Thrones. It's stuff which happened in the past which put these characters in the position they're in now.
What TWOK and FC have in common is that they're exciting and emotive films which produced a strong response in Trek fans and casual cinema-goers alike. That's really what made them hits, not the coincidence of deriving from old tv episodes.
Having said that, of course, I accept your point that being a sequel to an old tv ep need not turn off the casual audience.