The sport wasn't really young, it had been around at least 100 years (Compare that with how long MMA has been around) and contrary to popular belief, the technique was not crude, however it was different because boxing had just recently introduced gloves, gotten rid of wrestling throws, etc (Broughton rules), still wasn't too divorced from street origins, and fights were still often to the finish with dozens of rounds, etc. You'll notice that alot of what works in boxing, simply doesn't show up in the MMA prize ring. I think their hand skills are destined to mirror the techniques of early glove and bare-knuckle fighters.
Jeffries wasn't a skill fighter per se anyway, but of course if he were around today he'd have the access to all the skills he needed to maximize his attributes, with modern training methods he'd be unreal strong. His only liability would be that his tremendous endurance wouldn't have the same value it did in finish fights-he could loose desciscions to "cute" boxers, but only if they basically got on their bicycle.