exactly, it would be trivial to have a whitelist server side and now only ad friendly apps can access the videos. they only still work because it’s worth keeping those viewers in the system for the time being.

It's not trivial to make sure over the network on a device you don't control that you're talking with an app you think you are talking with. Just look how multiplayer games fail to combat cheaters and resort to kernel anticheats, and then still fail to assure the players are actually using the legit application. It's actually pretty much impossible in any open ecosystem, maybe possible on something like chromecast where you get to control almost anything (as long as someone doesn't hack it to run custom firmware, like they do with every console ever).

Not only is this impossible, it always makes the experience for your legit users worse (but hey, if they are fine with the level of ads on yt today they probably don't care if google were to mine bitcoins on their phones).