There's a niche into which the ARM-type MCU won't fit.

The ARM's I've seen so far are hobbled in performance when one attempts to put FLASH on the same die, because the technologies don't mix well. Likewise, putting a substantial amount of cache is already a challenge, and putting what amounts to a significantly useful RAM on a machine that has 32-bit words uses up lots of real estate. Consequently, while it's cheap enough for many controller applications, it's not cheap enough to compete with the really simple 805x or PIC or 68HC08 app's that operate entirely within the single-chipper.

Another factor is the performance. With the ARM's "reduced" instruction set, it takes longer than one cycle, to do almost anything, including things that would take only one cycle on a CISC type. Simple operations can fall into this class, though I'm hard-pressed to come up with one right this minute.

Certainly, operating on bytes can be cumbersome if your I/O is not in bytes, and the low-cost ARM versions don't operate on bytes any faster than, the faster 805x's. There will always be some things the little guys can do faster and more conveniently, and probably at less cost, particularly since the ARM types can't do much of anything without external memory.

RE