Punk losing to Brock at Summerslam doesn't mean the feud is over. It just means they'll book a rematch for later on and Punk could go over there. The whole idea is that Punk is the underdog against Lesnar and clearly outmatched by him in terms of size and strength and brute force, so him losing to Lesnar at least once before coming back more angry and determined than ever and defeating Lesnar the next go around is the best way to book the feud. Same thing with how Punk lost his first physical altercation with Lesnar, but came back the next week more confident than ever, challenging Lesnar to an official match at Summerslam.
Plus, even if Punk loses at Summerslam, that doesn't mean he hasn't been put over by Brock. If Punk puts up a hell of a fight and makes people believe he has the match won, then you can still consider Punk to have been putten over by Brock. You don't have to lose to someone to put them over, you just have to find a way to make them look credible. Punk making Brock look vulernable and maybe even on the edge of defeat would do that.
The last thing Lesnar needs is yet another loss without getting at least another win on top of it. If it wasn't for the fact that this feud is more about Punk/Heyman than Punk/Lesnar (defeating Brock is just Punk's ultimate way of defeating Heyman), I'd prefer if Brock won the feud period anyway.