The difference between Brock and Punk is that Punk wrestles on a weekly basis. Brock wrestles every so often, thus losses by Brock stand out more. I now remember that match with Triple H, very forgettable, and even then Lesnar didn't have a strong showing.(low blow by Heyman followed by a sledgehammer) To me, it just feels like Punk can keep his credibility based on his mic work and his weekly matches. Plus we all know this wont be a one off match, the only way WWE can logically make it a series of matches is to have Punk lose to Lesnar and prove all that stuff he says in promos about how Lesnar can beat him down but he;ll keep coming until he gets his hands on Heyman.
They could even throw Curtis Axel in the mix, maybe RVD and other Heyman guys as filler as Punk did say he would beat anyone who stands in the way of getting his hands on Heyman.