I think sub $1 Cortex-M0s like ST's STM32F100 are going to slowly kill the 8051s (also likely AVR and PIC) of the world. Hard to argue with the price and performance.

Using the same toolchain as higher performance MCU (Cortex-M4F) simplifies things because everyone hates bringing up embedded tools (debuggers, compilers, IDEs, etc) and enables re-use across product lines inside product companies. Factor in the increased popularity of the open source toolchains and their quality will improve as well.