All shows decline and fall.
Ahem, TNG didn't. If anything the ratings only increased over time right until the last episode. The little image above shows that.
I'm pretty sure Breaking Bad's ratings have only increased with every season, and the final half season is coming up next year.
There's probably plenty of other shows that never declined in ratings before ending.
(Not sure if what I'm saying really adds anything to either of the two sides to this little disagreement, but I still wanted to point that out.