I think what everyone was worried about back then was that Twitter was changing the nature of what Twitter was. Twitter started placing limits on API tokens, introduced new UI in the form of cards, which could also be used for ads, etc. There was a sense that the freedom and openness of the Twitter platform was quickly diminishing.
Twitter's response was basically no response, but in a good way. They slowed down making those sorts of radical changes, and to this day you can still browse Twitter with a 3rd-party app like TweetBot and never see cards or ads.
TweetBot is older than the API cap, so did they roll that back? It was 100,000 users (or 2x current users if that was >100,000 which TweetBot probably was). So if they didn't roll it back I guess TweetBot would have stopped adding clients.
(This is something that happens for me regularly -- I "discover" something only by its shutdown notice making the HN front page...).
Whenever stuff like this happens, I always wonder how many people, like me, hadn't heard of some service or product, and I wonder how many of them would have used it if they had. Would it have been enough to save the company from going under? Was it a marketing problem? I dunno, that's just the kind of strange places my mind tends to encounter in its errings.
Problem was none of them actually stopped posting on Twitter, at best they just set app.net to mirror their Twitter never really engaging with the network because none of them were willing to give up their high follower counts they had from being early adopters of Twitter.
Too much ego to put up with the BS from twitter, yet also too much ego to give up their high follower counts.