Obviously some people have experienced a change in their displays. It's been documented with both measurements and pics throughout this thread. However, the number of people who have this experience seems to be minute compared to the number of people who actually own the sets (as seen by the owners threads, etc).
It could be that the amount of black level rise varies from set to set. Or it could be that the people who don't perceive it simply don't have the kind of viewing environment necessary to notice it, or they aren't attuned enough to notice the change.
I consider it more likely that the vast majority of consumers wouldn't notice the black level rise due to the conditions mentioned above. Therefore Panasonic wouldn't consider it a problem they need to devote resources to to solve.