I really DON'T think most people were doing it to manage sensitive stomachs. It just became "the thing to do", without any real understanding of what potential problems legumes MIGHT cause. It is CLEAR that in dogs that HAVE DCM, taking them OFF grain free foods can immediately reverse their symptoms. SO there IS some connection. But that does NOT mean that it will CAUSE the problem in an otherwise healthy dog. It's like extrapolating that if salt is bad for people with high blood pressure, we should ALL strictly avoid salt. We now know that salt does not CAUSE high blood pressure, it's just bad for people who have it. (and NO salt is REALLY bad too!)I'm glad every time it isn't a blanket recommendation against grain, even if it's not a particularly nuanced response. It's definitely worth bringing up with the vet, knowing he did well on grain free in the past.
Among breeds prone to dilated cardiomyopathy, I wonder how many people were feeding grain free to manage a sensitive stomach and how that has been handled since.
Since getting ready to breed Panda, I've learned more, and I now know that a more general, and well-documented problem with grain-free foods is that the high levels of plant-based progesterone in the legume-based foods can interfere with the bitch cycling. So it has reproductive implications if you have breeding stock. I know that's not a BIG concern in this group, but a bit of information to keep tucked away in the back of your minds.