Well, Tapbots launched Ivory, a totally different app based loosely on Tweetbot, but it required a substantial investment to design and engineer. It's not like Tweetbot just started loading Mastodon one day. In fact, paying Tweetbot customers were left with a nonfunctional app, and many got prorated refunds from Apple.

Ivory has been well-received, but who knows what the numbers are compared to Tweetbot. So this was not a "just swap the API" slam dunk.