I was just thinking about this, because I used a whopper plopper again last Sunday for the first time in 2 years and caught my three biggest fish on it for that trip.
It is possible bass in some places have either undergone avoidance learning to the whopper plopper from being caught, or catchable bass have been selected out by harvest or mortality -- these would be the main mechanisms of reduced catch rates due to "fishing pressure". Generally, anything bass perceive as unnaturally-distinctive should be vulnerable to pressure -- the more they can tell the difference between it and typical food, the easier they will learn to avoid it. This is presumably why Spinnerbait effectiveness has appeared to wax and wane over time (Just google "are spinnerbaits dead?"), while things like worms, jigs and grubs really haven't.
But in order for an effect of pressure like this to be really noticeable for the WP, a very large percentage of bass in fishery would have to be affected in the short time the Whopper Plopper has been really popular... and that seems implausible to me except in small, very heavily-fished waters. The size and speed of the drop in effectiveness people are reporting seems to me unrealistic to be due entirely to fishing pressure.
On the other hand, there are plenty of people who have never found the WP very productive at all. They tried it, never had much success, and have never really understood the hype. And then there are people like me who often catch "a few" with it, who use it situationally, but have never had any of these big blow-out days where bass are jumping on it every other cast, and haven't really noticed any change in effectiveness. Some of the waters I fish get more pressure than others, but none are "untouched". And it happens that those where i have had the most success with the WP are among the more pressured.
This leads me to think we're mostly dealing with something that is more perception than reality -- in particular, an instance of Regression to the Mean.
Initially when the WP became popular, some people did really well, some did not, but mostly due to random factors. The big successes got a lot of attention (helped along by Chris Lane's Toledo Bend win), the failures did not, creating the perception of something more "magic" than it really was, and the WP blew up.
But because of the law of averages, random variations cancel themselves out over time, and those who did really well with the WP initially have just came down to earth, as they should if initial successes were due in a large part to healthy contributions of random chance.
And if you never had success with the WP initially, you wouldn't be fishing it now. So the opportunity to observe random swings of changing success in the other direction does not arise.
I would bet what people are experiencing here is maybe some fishing pressure, but only on top of a larger degree of regression toward the mean. Unless you're in small, heavily fished waters.
