Analysis / It's Popular, Now It Sucks
To the extent that this trope is true, it will usually be as a result of the non-lethal form of The Firefly Effect, where the studio are trying to boost ratings. There is one other way in which this trope can legitimately manifest, as well. a} An initial film (The Matrix being a good example, as was Cube) is released, which the releasing studio does not expect to be a mainstream success. Because the initial film is only expected to appeal to a niche audience, the storyline will often be more intelligent than usual, and the producers very often won't resort to the usual cheap gimmicks (excessive nudity or Michael Bay-like explosions unless those are genuinely in context with the storyline, etc) which are intended to draw in large audiences. b} Because of said film's unusual degree of artistic integrity, it will become far more popular than the studio anticipated. c} This causes a full-throated case of Sequelitis in the studio, due to the hope of making more money. Said sequels, unlike the original, will use the usual crowd-pleasing tropes mentioned above, as well as re-tooling the original film's premise to appeal to fifteen year old morons.