If beating GSP was this easy than he wouldnt be champ. The great bjj guy he already beat in Shields and he also beat Koscheck, the great wrestler you refer to. The problem with great strikers is that they often have poor ground games which makes it easy for GSP to just take them down for 5 rounds (Dan Hardy). The closest guy to having the skill set required to beat GSP would definitely be Nick Diaz. Better stand up and a good enough ground game to possibly be able to sneak in a submission.
Everybody knows GSP has great wrestling but the reason he wins so many fights is because his secondary skill is way better than the secondary skill of everybody else...until someone comes along who can change that I cant see GSP losing.