I think you have to step back and wonder what "growth at all cost" can turn into. Multiple research studies have confirmed Instagram, relative to other social media apps, has basically been engineered to maximize FOMO and is making people more depressed, isolated, feeling "not good enough", and in a constant cycle of comparison with others.
When you create an app that is as consequential as Instagram is on society/culture/media I think any smart person would have to step back and have that "what have I created" moment.
I have no clue why they are both leaving at the same time but I would imagine a health dose of reflection is in both of their futures.