At 10x the price of ATtiny85 this is why I say at the low end the price/performance is not optimal for Cortex-M.
What kind of pricing are you getting there?
This is an important question that you seem to be avoiding answering.
At Digikey, I can get an ATtiny85 for about $1.40 (q11) (seems "too expensive" to me, but that's what I get.)
In similar quantities, the follow ARM chips (with 32 pins or less) are cheaper. All have faster clocks, more memory, and (arguably) better peripherals (ie a UART!)
NXP (freescale) MKL03Z
Atmel SAMD09D14
NXP (freescale) MKL02Z
SiLabs EFM32ZG108F16
ST STM32F030F4P6
NXP LPC812M
SiLabs EFM32GZ110F16
Nuvoton Mini54
there are some more (including 8pin chips) if I reduce the requirement to 8k program space (like the tiny85.)
As other people have pointed out, the pricing issues get (even less clear for bigger parts. The cheapest AVR I can find with 128k of program memory is about $7 (and frankly, the AVR architecture starts to get ugly above 128k.) There are 25+ PAGES of ARMs before I hit that price/memory combination. In fact, the cheapest (about $2) has 256k of program space.
I like AVRs. They are simpler to use. But the added complexities of an ARM are hardly insurmountable!
Now, the vendors that market a 8k ARM to replace a 8k 8bit cpu worry me, because a lot of the code that one puts on a small 8bit cpu is NOT the sort of thing that gets smaller because you have a 32bit micro. But I don't have much fear that a 16k ARM won't work. And it's getting to the point where that WILL be the more economical choice.