You're talking as if you've stepped in the ring with the GGG and came out the same way you went in.
GGG held a significant weight advantage over Kell Brook, in addition GGG has a KO% of around 90+; when you put that into perspective and look at how Brook was taking those punishing blows then you'd understand that it was the perfect stoppage from his corner because Brook was never going to quit and he'd risk serious damage to his health. On top of that he had broken his eye socket, it would have been nonsensical to continue.
Am not sure you understand how Boxing works, it's not a fight till the death
I think the only people we can call cowards are keyboard warriors not boxer's.