An interesting thing to ponder is what would have been Star Trek's fortunes had Jeff Hunter had gone to series? Hunter passed away May of '69 (at age 42) due to complications from a fall and a stroke.
Would TOS had been as appealing with Captain Pike? Would there have been conventions or the novelty of mass adoration that marked the 70s? Would it have had the cinematic second-wind that TOS enjoyed without the original captain? Would there have been a TNG?