That's not really how trauma usually works - it's way more complex than that. If you've experienced something traumatic and it has had longterm mental and/or physical effects on you, reversing or undoing the situation won't just magically cure you of those effects (and sometimes going back to the way things were before might not even be the best option for you).

Compare it to a bad break-up: you got cheated on and dumped and now you're dealing with the psychological consequences (the pain of being left and betrayed, trust issues, the loss of your partner, etc.). Now if your ex apologizes to you and you decide to get back together, that most likely won't magically cure you of your trust issues or undo the pain the break-up caused you - and maybe getting back together with your ex isn't what's best for your mental wellbeing at all, even if you miss them a lot and wish they had never betrayed and left you in the first place.

My point is this can be a psychological/psychosomatic issue (or at least in part) caused by Richie leaving, but that doesn't mean that Richie coming back would magically undo it, and Jon might also feel like Richie coming back now really wouldn't do his wellbeing any good at all.