We all know that the professional MX/SX racers drive the market in this sport. Whatever they do, we all follow. And no professional is going to knowingly ride a 2T that has the same or less power output as a 4T, because it isnt as usable, so they're at a disadvantage. The AMA caters to manufacturers by making rules that forces amateurs to follow the trends set by the pros if they want to be competitive amongst their peers.
We as consumers can change the AMA's mind by showing the manufacturers that 2 strokes aren't dead. Keep buying 2 strokes. The AMA amateur rules are helping us, as the amateur displacement rules which allow for larger displacement 2 strokes to compete against 4 strokes (150's in the 250 4T class, etc). So we can still buy 2 strokes and be competitive.
Right now, KTM is doing the 2T a HUGE favor by selling 150 SX's and 300 SX's. Depending on your local rules, they're allowed in the 250 and 450 4T classes, respectively. As the 150 and 300's become more popular, the Japanese manufacturer will sell fewer 125's, and will have to build 150 and 300's that compete with KTM. Then you'll see more of a split between 2T and 4T on the amateur starting lines.
Then inevitibly, more and more young professional riders will come up through the ranks, racing 300's, and the AMA will have to adjust their rules.
It might take 5-10 years, but I think the 2 stroke will be back in AMA pro racing.