It's hard to say just what would have happened. Kylo had barely been in his right mind at that moment, even before the betrayal that had set him on a mission to personally destroy every last bit of the Resistance if he could. But if she had stayed? If she had given him what he'd been hoping for, rather than doing the exact thing he most feared?

"I wouldn't have chased them." He can say that much. His thought had been to let things go as they were, to let the Resistance die or get away as they managed under the current circumstances. Whoever did escape, he wouldn't have pursued, especially not with her there convincing him not to. It's easy to imagine that if she had stayed, the battle on the surface of Crait might not have happened at all.