This story at Uber is a fascinating example of the growing pains involved with transitioning to a new language.

LinkedIn adopted Swift back in 1.2. I would love to read some war stories of how things there have progressed over the years.

Source: https://www.youtube.com/watch?v=X9waDi787uo (14:15-15:30, 39:26-40:37)

At an interview with them a few years ago, I was told they had been moving towards rewriting the app in Objective-C.