It would take years and years to lose the "big fish" genes from from the entire population. This is not a quick process.

Also, fish an deer are somewhat different because most fish grow continuously throughout their lives, but deer stop growing larger at physical maturity. So in some regards, rather than worrying about preserving "lunker genes" it is more important to maximize fish longevity. That is probably the reason for the 12" limit.

However, on the flip-side it is possible that the the 12" fish could overpopulate and consume too many resources, thus limiting growth rates and the number of "lunkers" that the lake can support. Seems like I read somewhere that lakes with good bass populations will often produce larger crappie because the bass eat more middle-size crappie, freeing up food for the bigger crapie (This last statement may not be true...not sure).