But the happiness at the right end of the graph will never go back to 100 (as in 100% of the pre-smartphone happiness, I think they mean).

There are instinctive good feelings about interacting face-to-face with people, especially while doing an enjoyable shared activity like eating. There are provable health benefits from spending time near, or on bodies of water and I'm guessing also just from spending time in the woods. There's something about staying out just a little longer to be able to watch the sunset.

No matter what that phone screen allows you to interact with, it can't provide that stuff. So yes, happiness rebound. But not to 100%.