When laid out on a timeline you can tell that the problem occurred very early, as I never caught bass at relative weight or better. This leads me to think that there was never a good effective spawn, and the bass eliminated what was there, leaving me very few bream to maintain the aggressive bass population.
The only question left with that scenario is where are the 19 month old bass? 13" is about the max that we have seen, which would equate to fish from 2016's spawn.