It's kinda a neat idea for a fixed target CPU like on a games console, but for a general purpose CPU range you generally don't want to reveal too much behind the curtain like that, what if you did a new model with a bigger scratchpad? Would existing software just ignore it? Or a budget model with less? Do you have a crash, or just an slow fallback? The system where the CPU magically makes cache work is better when you're in a situation where the CPU and software problem aren't fixed.