There's legal precedent already in Europe that this is fine by GDPR rules, as long as the price of the subscription is "reasonable".
That way they get to preserve (or even improve) their ad targeting business, on the assumption that most users will just choose selling their data over a subscription. And if they go for a subscription, even better. In a sense, let the market decide the value of privacy.
The first step in this would obviously be killing any third party alternatives that would be the first place of refuge when they make that move.
In any case, a pet theory, but there's been a strange convergence by these big companies and the way they're changing their business models.