IMHO a pH crash is a complete myth. pH is simply a symptom and adding CO2 will lower it. Adding 30ppm CO2 will cause the pH to drop by about 1 degree, regardless of the starting pH, and the fish are completely unaffected. What you want to avoid are huge, rapid changes in the TDS IMO.
I've never heard of an authenticated case of a pH crash really causing problems (and I've run 0dKH tanks for years, often with pretty slack maintenance if I'm to be honest and even down to pH values of 4.2) but heard plenty of people blaming a pH crash of killing their fish. When you look beneath the surface something has happened in their tank like something being added, too much food being fed, not enough water/filter cleaning or similar. The thing is the first thing people do is reach for the test kits and the most common one is a pH test kit and something goes wrong, they test their water, find the pH has changed so blame the 'pH crash' as killing their fish! That's like saying 2 + 2 = dog!!!! There's no proven link.