It would put them out of business, or fundamentally change the business model to one where engagement is not the primary metric for users. These platforms are

It would put them out of business, or fundamentally change the business model to one where engagement is not the primary metric for users. These platforms are so profitable because of targeted advertising. And most of the harm of these platforms comes from privacy invasion (tied to advertising) and the need to juice engagement numbers by manipulating emotions (also tied to advertising). A subscription model would not have the same problems. There are serious first amendment concerns with attempting to regulate the presentation of websites and apps.