I didn't read all the pages of this thread, so this may have been said but, you can't judge the quality of a show simply by viewership ratings.
And when it comes to viewership ratings, there is such wide plethora of complex factors, like time slots, markets, competition, promotion, et al. Looking at a seasons average ratings is not necessarily helpful either.
Premiers, season premiers, season finales, and finales will garner higher rating because they are heavily promoted. Likewise for big budget 2-parters. TNG began with wide coverage, good time slots(depending on where), and almost no competition. I think the show caught on with general audiences. Even at the height of TNG(around season 6) it was still no where near the most popular show on TV.
Interesting facts:
Cheers(NBC) premiered in 1983. It's first season achieved ratings similar to TNG (10-13) and these were considered lousy. The second season saw ratings in the 20s and received ratings in the upper 20s in most of the later seasons. This was considered very very good.
The finale for cheers was watched by like 90 million people in 1993, the 11th most watch TV episode in history.
The Office premiered in 2005 and its first season averaged ratings of around 5 million people. This was considered lousy.
In 2012, The Office(in its final season)averaged about 4.2 million viewers. In 2012, The office was the highest rated sitcom on NBC!