Everything in moderation, the popcorn lung was found in people after they had worked 8-10 hours a day 5 days a week and exposed in EVERY breath they took. That's far different from what we, as vapers, experience to say the least. Is it bad, sure it is, but let's be realistic here for a moment. We inhaled far worse from analogs and there's ZERO chance we would get anywhere near the diacetyl unless we chain vaped it at the same exposure duration(years) and amounts, which we dont.
Breathing in some major cities is at least as hazardous as vaping and truly the only safe thing is not inhaling anything but fresh air, far from cities, cars, factories, etc.