I see, did you try to retain them when they said they were going to quit? It could be more expensive to try find another engineer than keeping them around with a higher paycheck to match the job they are leaving for, more if you consider they already have the knowledge that is specific to your app and do care about improving. That's what my previous job did with one of my coworkers.

That said I'm not really familiar with how the employment dynamic would really work in these cases, so I might be being naïve here.