I think what Erik and I both mean is that new designs going forward will use the ARMs instead of the 8051s. There's no real cost difference in terms of board design or power-supply requirement or the like.

There are a few small things where the C8051F314 is a good little processor, and at less than $3 each single quantity they're pretty great, especially for a sharp-pencil BOM where the dollar between it and the SiM3C134 is important.

But unless your existing design's processor has been EOLed by the vendor, there's no reason to change the design to use a newer part.
-a