No, we don't know. Just like we didn't know how Trump's blatant racism, idiocy, and inexperience would play out.
Like, is there evidence in the polls that suffering a heart attack was a significant setback to Bernie's chances? Is there any world where Trump's health is actually a competitive advantage, considering his age, his lack of discipline, and all that other stuff? Just as Bernie's heart attack helped to open my heart to him, could we not imagine Bernie using an event like that to his advantage, conjuring up sympathy, galvanizing a sense of collective purpose, using it to create a point of contrast between him and his opponent?
What I'm trying to say here: this rush to defeatism in the face of a significant but frequently workable health issue isn't rooted in anything concrete. Trust me, I understand the fear; health issues aside, even, it'd be nice for the average age of our candidates to be significantly lower. The process of rooting for a candidate always includes making a gamble, though. I don't think Bernie's heart attack makes him particularly more vulnerable to Republican nonsense than the endless other things that have been brought up about all the other frontrunners.