@Josh Barnett is pretty active in here, so it'd be cool to get his thoughts. Ken was so strong and had such a suffocating top game, and he was insanely fast for his size, but Josh is better from more positions and has a wider array of submissions. It's hard for me to imagine Ken losing to anybody in his prime - and other than the worked losses to Funaki and Suzuki and the disastrous Severn rematch, he didn't lose to anybody in his prime - and if this fantasy match-up happens in Pancrase then it's literally impossible for me to imagine Ken losing, but if it happens in the UFC then Josh's chances increase with his striking in the clinch on the feet and his GNP on the ground. If we're talking pure grappling like ADCC or something, we never saw Ken compete with that rule set but I'd still give him the edge for his top game and his scrambling ability. Definitely one of the more interesting "What Ifs?" in relation to Ken.
In what way isn't he? Stronger, faster, better TDs, nastier leg locks, higher percentage of submission finishes in MMA. Head-to-head, it's interesting to consider, but anyone who knows anything about catch wrestling or grappling in general knows how elite Ken's submission game was.