I don't think your data sample is the correct size or in the right place over enough time.
There's a variety of reasons that could cause folks to sell their M&P, but I don't think "because it isn't a good gun" is one of them. More than likely it has to do with their increase in popularity and, like Glocks, their increase in being bartered back and forth. There are A LOT of M&Ps out there so it reasons that it's more likely to find one for sale than a rare vintage piece.
There's a variety of reasons that could cause folks to sell their M&P, but I don't think "because it isn't a good gun" is one of them. More than likely it has to do with their increase in popularity and, like Glocks, their increase in being bartered back and forth. There are A LOT of M&Ps out there so it reasons that it's more likely to find one for sale than a rare vintage piece.