I agree with everything you said. I agree that people who haven't been Ko'd can always be ko'd especially when facing a vastly superior striker.
The thing is I don't think Guillard is a vastly superior striker. Sure Guillard can Ko him if he catches him clean but he's not as good a striker as people believe. His athletic ability, speed and power make up for a bunch of defensive flaws and his chin is very suspect.
I just have a hard time picturing a guy who got out boxed by Stephens, Lauzon and Cerrone out boxing Varner. Especially when Varner is a better boxer than all three of them. Guillard is a front runner and once he figures out that Varner won't fold and will land hard shots on him also, he'll fold like he usually does, and yes Varner also folds but usually only when being forced to grapple with far superior grapplers.
It's not that Melvin isn't very dangerous he's just a less skilled fighter than Varner and I rarely ever bet on someone whose only chance is a Ko punch. Especially when he's facing a more technically skilled striker who has a far better chin. Melvin's chin, striking defense and willpower will lead to his downfall imo.