Twitter's rate limiting has been reported as perhaps being a failure to pay bills or otherwise properly manage their servers, and not some specific policy change. So that particular example might not be what you were focusing on or what you meant to focus on. Obviously Twitter made many other decisions, and the recent big one is cutting off access to tweets for people who are not logged in.
As for Reddit, the price of the API is not the point. Rather, the price is so high that nobody's going to use the API, and that's the point. But they want to pretend that it's still possible to be used. And we know this is true because if the API really has such high value, then presumably some of the popular clients out there would have been worth it for Reddit to purchase, and the purchase price would presumably have some correlation to API usage.
On a more general level, though, I think what you're talking about is the process known as "enshittification". It's possible for social media companies to avoid this end result, but it requires great care especially in the early stages.
Burrbromb