This graph is misleading. It is on a decline because of streaming services and youtube, not because he was losing popularity or anything. In fact, viewership for Kimmel actually exploded on Youtube in the early 2010s, which is contradictory to that graph you provided. Google it for yourself if you don't believe me.
It’s a mix of two realities: cable viewership is declining across the board, but in Kimmel’s case it’s compounded by the fact that more and more people have figured out he’s just a smug, unfunny douchebag. Nothing of value was lost.