Comparing the popularity of TOS and TNG is impossible, since both the TV and movie industries were undergoing change during those years (and that change of course still continues).
In TOS' day, there were three networks, that's it, and any show got heart-attack ratings by the standards of TNG's day or certainly, now.
But as TV was balkanizing, movies were consolidating. Now, any big blockbuster movie must be HUGE to merit the budget Hollywood puts into blockbusters. Mid-range movies have vanished.
This means that in the case of both TV and movies, no good comparisons can be made over time, going by ratings or box office.
The only sensible comparison can be made between TOS and TNG (or any of the other series) in the realm of pop culture. And there, TOS is the clear winner. The TOS characters are iconic, which is why JJ Abrams found them worthwhile to recast. Only Data and possibly Worf is iconic in that way, and I don't expect to ever see the TNG characters again.
The next series will have a cast of all-new characters, possibly with one memorable character recast for old-time's sake.
If nothing else, Star Trek 2009 showed CBS that their Trek back catalog still had substantial value.
It did a lot more than that - it proved the franchise has audience appeal and is not just a lot of passe old crap. That is by far the most important thing Abrams has done. I liked the movie, but even if I'd hated it, I still would have appreciated that. I want
Star Trek back on TV, where it belongs, and for the taint of failure to be removed is the first necessary step in that process.
And if it wasn't popular any more to begin with, how come they managed to make Star Trek 2009 a success?
I know that's rhetorical, but it's also a crucial point. S
tar Trek is not popular or unpopular in a vacuum. It's popular or unpopular according to the medium in which it lives. It failed on both TV and movies because it didn't follow the trends of TV or movies.
Now Abrams has come along and refurbished it according to what is required for a big-budget popcorn movie nowadays. That still doesn't imply TV success, since TV and movies are completely unlike each other now. A successful
Star Trek TV series will need yet another makeover, even bigger than what Abrams did to make it work within the context of movies.