True, the Horsemen have generally always been made up of at least one young upstart, some promising young guy with tons of potential. The problem was that Paul Roma wasn't really a young upstart, nor was he someone that fans saw having a ton of potential. He may have started out that way once upon a time, but that's not ultimately how fans would wind up seeing him circa 1993.
Paul Roma's wrestling career began in 1985, he had a tremendous look and a lot of all around athletic ability. In no time, he was signed to a deal in WWF. He & Jim Powers formed The Young Stallions and they got pushed for a while, though I think one reason why the team didn't go far is because Powers & Roma didn't get along. Another reason, allegedly, is that Roma already had a huge ego and wasn't shy about letting anyone know it. He hadn't paid any dues, yet, reportedly, thought he warranted some sort of special treatment.
By the time he popped up at Slamboree in 1993, Roma was no longer this hot, up & comer that may have had potential. He was someone that had just turned 33 years of age slightly more than a month earlier and he was just someone who'd spent 6 years in WWF as a jobber. If WCW wanted some young upstart to fill in a role as part of the Horsemen, maybe they should've tried making a serious play in approaching someone like Shawn Michaels or used someone already signed to the company at the time like "Stunning" Steve Austin.
I don't necessarily think Orndorff would have been a particularly great choice, but he certainly had more star power and was a bigger star than Roma had ever been. However, by 1993, Orndorff was really past his prime. He was still in great shape and could still go, but he was pretty close to age 44 by the time Roma was revealed as the newest Horseman. Even though he was a bigger star than Roma, Orndorff wasn't really someone that would've made an impact. Orndorff's biggest days as a star were well behind him as his feud against Hulk Hogan took place during the mid 80s.