Pulling the sort of crap that modern websites pull, the little FOMO nags or notifications or emails about "Please don't leave we're desperate and you're really cool" or whatever, is a great way of filling your site with people who don't actually want to be there, and that's a one-way ticket to a dysfunctional, unhealthy community.
Hold people GENTLY. When it's time for them to go, wish them well and let them go, don't try to stop them or lure them or entice them back.
