Even that is probably a simplistic reading. A lot of people watching TNG weren't watching it because it had Star Trek in the name, but because it was practically the only show of its kind on the air, and many people still watched network television exclusively. By the end of TNG's run, Trek had started competing against itself, and there were a lot more sci-fi/fantasy shows on tv.
Many people who aren't Trek fans and don't even know much about the franchise are familiar with TNG (same goes for TOS, but mostly through movies rather than the show), which suggests that TNG was a ratings success not because it had a huge built-in audience, but because it attracted a lot of casual viewers and non-Trek fans. Most of those people didn't watch every episode of the show, they watched some of it and are familiar with the characters (Picard, Data...).
Now wait just a damn minute, as Dr McCoy would say. Although scifi's taken off since, in part due
to the success of TNG, there was still
quite a bit of science-fiction on television at the time
. Why weren't the other series as big a success? TNG was a hit not because it was the only TV of its kind but because it was great TV period. Credit where due, people.