Have you let Nikon know about your problem/issue?

The D90 is still being sold and used heavily by both professionals and amateurs (and likely making better images than people bragging about spending $3000 to get more metal in their cameras).

Let's make things simpler. Take two events: A and B.

If it's true that A implies B, then if A happens, B must also happen. Also, if A happens, and B does not happen, then that implication (A implies B) is invalid.

Let A be plastic construction in a camera. Let B be internal dust buildup (without external influence).

There are many camera models with plastic front plates and no dust problems. (A is true, and B is false).

Therefore, plastic front plates do not cause dust problems. (Therefore, the statement A implies B (or if A then B, or A causes B) is false.)

Also, lets take a hypothetical case, and say that the D7000 and D600 were the only cameras with plastic front plates to ever be manufactured (i.e., no P and S cameras exist, Nikon has nothing other than the D1/2/3/4/200/300/700/800, and same with every other manufacturer). Then you might be able to show correlation, but correlation doesn't imply causation.