Actually, i feel like they are even obligated to change into new PB's, or at least the new G5 processors.
Who would buy a G4 powerbook now?
For the price of the new PB4s, it would be bad to buy one now, than see them change in a few months.
So... i think people would wait for the new ones anyway, and Apple probably just realise that to.
It's just a logical next step.