Sonak, the only difference is that TOS was a Network show, in prime-time at a time where little else competed for attention. Yes TNG was as popular as any syndicated sci fi drama has ever been, but that is 'almost' nothing compared to any Network show then (and I'm afraid) now.
Not true. Because of what's happened in the intervening years, people forget just how staggeringly popular TNG was in its heyday. Even though it was in syndication, it was routinely pulling ratings numbers that beat major network competition. At one point, it was routinely beating
Monday Night Football in the ratings.
EDIT to Add: By way of comparison, I pulled the viewership numbers TNG was getting in its prime. During the 4th, 5th, and 6th seasons, TNG was averaging 10.58, 11.5, and 10.83 million viewers respectively. By comparison, over those same three years,
Law & Order, which was a powerhouse for NBC, was averaging 12.0, 12.3, and 10.5 million viewers. Yes, in season six, TNG actually bested a hit primetime network show in average viewership, and in seasons four and five, it wasn't far behind.
Did TNG draw the ratings of the top shows like
Friends? No. But it held its own with the networks during its run, and in fact beat many very successful network programs that were considered hits. And I would venture to say that, in its prime, had TNG actually been on a network in a stable timeslot, instead of syndicated all over the place, it might just have made it up to the very top.