EXACTLY. I agree with everything you said. There hasn't been any testing. There isn't any true evidence.
And yes, the logic behind the side loading is sound. The thing is, why do some people believe that there is some magic line drawn between a 3.250" stroke and a 3.400" stroke? As if the 331 will last forever and the 347 will spontaneously combust at any given moment. There is less than 5% difference in displacement between the two engines. I'd laugh if there was a test done and it was determined that there was a 5% difference in longevity, and the 331 lasted 100,000 miles but the 347 only made it to 95,000. It's a completely moot point, because you know neither one is going to last as long as a 302/306 would. If someone wants to cry about longevity, then don't build a stroker to begin with. Buy a Honda.
The funny thing is, a lot of the people who build 331s will take them to the track and absolutely beat the car like a scalded dog. And they're concerned with reliability?
I dunno, that's my take on it. I'll take a definitive power advantage over a mythical reliability advantage every day of the week.