I've witnessed school children formulate better arguments than this.
If all Pop has to work on is his pass coverage, then I say keep him in and work on it. I'd give him a few more games.
I also recall that it's been mentioned in previous threads that a few of the 'blown coverages' were zone and manuel didn't offer the required deep help. I think Manuel is the problem with our secondary, not Poppinga.
I would like to see Poppinga challenge receivers at the line and interfere with their routes within the first five yards more. Maybe he doesn't have the speed to recover, but that's not what I've seen. I think he's fast he just doesn't get between his target and the Q.