Khan is the reason the series is still going. If #2 had stunk as bad as #1 there would likely never had been a #3 or Next Gen, etc, etc.

And because of the greatness of Khan, fans are still willing to forgive stinkers like #5 and continue to give the franchise another chance. It is the "there is always a possibility that the next move will be another #2."

IMHO, without Khan, Star Trek would have died out by now.