for example I think feature length films that YouTube sells (or rents) already use this encryption.

That’s why authors should pony up and pay for the encryption feature and rest should be free to download. YouTube could embed ads this way too.

That's a wildly imaginative fever dream you're having. There is no timeline in which content creators would pay YouTube to encrypt their video content.

Here's a thought: what if paying a fixed amount to encrypt your video would grant you a much higher commission for the ads shown?