But it's not really comparable. TNG was first-run syndication, where the competition was considerably less than it would be in a network prime-time slot. TNG, like TOS, never cracked the top 30 in ratings, and it is highly doubtful it would have lasted as long as it did on one of the big four broadcast networks.
And not to mention TOS was a huge success when it reruns went into syndication too. I am pretty sure the series would not have been cancelled if they knew what was going to happen later.
They didn't continue the franchise in movies out of the goodness of their hearts.