In the internal email from Sep. 2018 that gave the $270 per teen "lifetime value" (LTV), the author also says that it's critical to keep this number in mind because you don't want to spend more than the LTV of the user attracting that user or providing anything of value to them.
So Meta's thinking is 100% on Cory Doctorow's enshittification journey because they want to spend the least amount of money possible on supporting their ad-supported teen and pre-teen audiences across their whole lives.