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).
At the scale youtube does things it doesn’t really cost them much. They actually have the servers, the bandwidth, often the power. It’s not like they get to sell a server if enough people leave. The savings probably do not even justify the effort (=cost) to unplug the server. And they still get many other benefits of having you as a user (getting to profile you, getting to push propaganda, getting to sell your information, maybe you send videos to friends who don’t use adblock, maybe you buy merch from creators making creators happier on their platform).
On the other hand this enshittification is ruining their monopoly, other areas of business (if you don’t need youtube, maybe you don’t need a chromecast? If you don’t need a chromecast, would you buy pixel phone, that can only do chromecast if samsung can do hdmi? Maybe you’re done with android? And if you’re on iphone, are you still using google maps, photos, search, keep etc?) and curing people who are addicted to the platform.
The eventual consequence of enshittification is always platform death. Which would be amazing, sadly the next platform is going to repeat the cycle.