If his UCL is shot it's going to happen anyway - better sooner than later.
If there's nothing with the UCL, I don't see any obvious explanation for why this has become such a chronic issue. Hopefully there is one, and it's fixable short of surgery.
A lot of things can cause elbow pain that aren't related to UCL. The recovery rate for a second TJS is below 50%, so him undergoing a second TJS is close admitting he's done as a pitcher. The Cubs should do everything possible to avoid it. Even if he had TJS tomorrow and returned it wouldn't be until 2020 when he's pitching again, so there isn't a rush.