This sort of thing was exactly the sort of reason why I was hesitant with buying Tweetbot. I understand that it's a superior client to Twitter's own native client, but while I don't have a lot of issues with the native version, the fact that they killed off their macOS version is completed unreasonable (especially since the Windows version was recently relaunched). Also, Twitter's stance on developers has always been flaky, so this seems to be only a matter of time.
I don't know what will force them to change their mind, but I guess there's nothing anyone can do at this point.
Funnily enough I had the opposite reaction, this was the reason why I was even more determined to buy the latest version of TweetBot for Mac on day one even with this threat hanging over them. As Twitter attempts to squeeze 3rd parties into oblivion it is even more important to support them.