I have seen literature that shows only a 4 year average life span for bluegills. If you do not have recruitment of several ages classes (which you won't for hybrids) it's no suprise you would have a sudden lack of large bluegills if they were dying out.

Even with bluegills that are not hybrids I have seen fisheries survery reports predicting a decline in bluegills in future years when there is a abundance of larger bluegills but not much recruitment.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.