TNG and DS9, etc. gave us all the proof we need that STAR TREK can thrive without the TOS cast. And Nimoy and Barrett would've likely been on board for COS anyway...
I FULLY disagree with that assessment as IF TOS had not become as popular as it did; TNG (as realized in its first season in 1987) woiuld NOT have survived past its first season; and while it did eventually gain an audience (around mid season 3); it survived past seasons one and two ONLY becuase TOS had been so popular, and TOS fans were doing all they could to make sure 'Star Trek' got its second chance.
I think a lot of fans don rose-colored glasses in regard to early TNG and tend to forget how much its first two seasons were panned by Star Trek fans and critics alike. Again, TNG's tide turned in season 3 and it was
Yesterday's Enterprise and the
Best of Both Worlds cliffhanger (first time Star Trek ever did something like that), that brought new viewers and started to get positive press from critics.