You're right, but the price drop is not supposed to boost Core or Elite sales, it's supposed to boost Premium sales because that's what they're trying to clear out, as relayed by mrstickball earlier:

"this source at MS has stated what I've been saying since the drop was announced: They are clearing out inventory of non-Falcon models. Once the Falcon launches, they are going to drop the price another $50 on all models. Not only this, Microsoft will be MAKING money on the $299 premium. This is the exact reasoning why H&E said they would make the X360 profitable by Q4 2007 - a $299 premium that makes MS money.

He also said pricing on the Core/Elite was not taking a $50 drop. It was going to be $279 for core, and $449 for the Elite, as they are not as popular (production-wise), and far less inventory to clear."

Original NeoGAF comment from 7/29.

Again:

This price drop = clearing out inventory.

Core/Elite = far less inventory to clear.

One of my best friends is in management at Best Buy and we were discussing this yesterday. He basically said the same thing, he though that MS, like Sony, was trying to clear inventory. It seems MS has stuffed the channel to get rid of all of the 360 with the older designs to prepare them selves to manufacture the Falcon units. All that said their price reductions won't do much in my view. The Premium still isn't close to mass market pricing. The Core is more worthless than ever and the Elite is still way over priced.

I just can't see any reason for MS to screw their fans that badly. Really, the last thing they need to do is make Halo 3 fans think they bought a more expensive, lower quality machine. The backlash would be insane for MS. If all they wanted to do was clear out stock so they could make room for the new, cheaper, machine they'd just lower the price to that point.