TOS and TNG aired in a completely different tv envinroment than Ent - far less competition, far less market saturation.
If they were launched today, TOS, with its '60 effects and, in part, stories, would not last a season, while TNG, with its sub-par first two seasons, would not see a third.
Today, neither series would spawn anything close to the fanbase they created.
I already said it - the first two seasons of TNG are weaker then Ent's fist two seasons; and season 3 and 4 of Ent can compete with every TNG season you care to name.
We'll never know what ENT would have been like if it had 7 seasons. But if we compare only the first 4 seasons of both shows, they're pretty evenly matched. And yep, as bad as ENT S2 was, it's nowhere as bad as TNG season 1. Nothing in Trek is as bad as TNG season 1. And season 2 isn't much better. "Up the Long Ladder" may the worst Trek episode of all time - and its only rivals are some of the TNG S1 episodes ("Code of Honor", "Angel One", "The Last Outpost"...)
And you know, just like ENT had T'Pol in a catsuit... TNG had Troi in a bunny costume. So why is one bad and the other isn't? (Not to mention that T'Pol was IMO a much better character and never as useless as Troi, particularly early seasons Troi...)