Instead of aggressively monetizing "engagement" with advertising via physiological engineering and manipulation. Matching each person to the most addictive content, customized to that person's mental weaknesses.
I.e. creating and leveraging mental health issues is their goal.
1. The value of a social media site is directly proportional to the number of people you want to engage with who are on the same site, and hence directly proportional (probabilistically) to the total number of users. A free site is always going to attract more users than a paid one, so a paid site will always be at a competitive disadvantage. Because social media tends to be a winner-take-all economy, no paid site will ever survive.
2. Different people want and need different things from a social media site, and so no one policy is ever going to make everybody happy. For example, the needs of people who want free speech will always be in conflict from the needs of people who want to filter out falsehoods.
Other than that, it's a cake walk.
> charged a small monthly fee
These two goals are diametrically opposed. Social media sites only hit critical mass when everyone has free access to them.