>At the same time, YouTube videos are getting longer, and people are watching more YouTube videos on TVs than on mobile devices

I wager most people are putting those on while having a meal and using their phones or tablets at the same time. Moreover, 99% of the most watched content on YouTube is utter garbage that would make the average reality show on TV twenty years ago look like The Godfather in comparison. Gossippy, clickbait videos made to induce an immediate dopamine dump and be used as background noise aren't "in-depth" anything. I don't think people are sitting in front of a TV watching an hour-long, non-sponsored, ad-free interview with Margerite Duras and doing nothing else concurrently, for instance.

On top of all that, this trend of making longer videos comes mostly from an attempt to increase ad revenue. Let's not be fooled here.