Ruud is a pretty good claycourter, but he really strikes me as a perennial top 15 or 20 type of player. Plus, as the other guy mentioned, he may have to contend with the likes of Alcaraz (who could well be a threat on the surface — time will tell) in the future. I don’t see him passing Thiem’s accomplishments on clay, though maybe he could wind up winning a Masters where Thiem has failed.
Thiem will be a higher level. He would just be stopped by Djokovic and mainly Nadal yet Casper when he is Thiem age won’t have to worry about Djokodal or any greats so he might end up with the better resume