The difference was ridiculous - we were actually porting a prototype algorithm from a powerful TI device with hardware floating point. It turned out viable to simply compile the same algorithm with software emulation of floating point - the Cortex M0 could keep up.
Having said all that though: the 8051 solution was so much physically smaller that the ARM just wouldn't have been viable in some products (this was more significant because having the analogue circuitry on-chip limited how small the feature size for the digital part of the silicon could be).
Obviously that was quite a while ago! But even at the time, I was amazed how much difference the simpler chip made actually made to the size of the solution. The ARM would have been a total deal breaker for that first project, it would just have been too big. I could certainly believe people are still programming for applications like that where a modern CPU doesn't get a look in.
But it's true that outside the top-level "don't make dumb design decisions" decision points, application code in the embedded world is reasonably insulated form this kind of nonsense. But that's because the folks you're standing on did the work for you.
if we can believe the datasheet, it's basically a pic12f clone (with 55 'powerful' instructions, most single-cycle) with 512 instructions of memory, a 4-level hardware stack, and 32 bytes of ram, with an internal 20 megahertz clock, 20 milliamps per pin at 5 volts, burning half a microamp in halt mode and 700 microamps at full speed at 3 volts
and it costs less than most discrete transistors. in fact, although that page is the sop-8 version, you can get it in a sot23-6 package too
there are definitely a lot of things you can do with this chip if you're willing to optimize your code. but you aren't going to start with a 30-kilobyte firmware image and optimize it until it fits
yeah it's not an nrf52840 and you probably can't do ble on it. but the ny8a051h costs 1.58¢, and an nrf52840 costs 245¢, 154 times as much, and only runs three times as fast on the kinds of things you'd mostly use the ny8a051h for. it does have a lot more than 154 times as much ram tho
for 11.83¢ you can get a ch32v003 https://www.lcsc.com/product-detail/Microcontroller-Units-MC... which is a 48 megahertz risc-v processor with 2 kilobytes of ram, 16 kilobytes of flash, a 10-bit 1.7 megahertz adc, and an on-chip op-amp. so for 5% of the cost of the nrf52840 you get 50% of the cpu speed, 1.6% of the ram, and 0% of the bluetooth
for 70¢, less than a third the price of the nrf52840, you can get an ice40ul-640 https://www.lcsc.com/product-detail/Programmable-Logic-Devic... which i'm pretty sure can do bluetooth. though it might be saner to hook it up to one of the microcontrollers mentioned above (or maybe something with a few more pins), you can probably fit olof kindgren's serv implementation of risc-v https://github.com/olofk/serv into about a third of it and probably get over a mips out of it. but the total amount of block ram is 7 kilobytes. the compensating virtue is that you have another 400 or so luts and flip-flops to do certain kinds of data processing a lot faster and more predictably than a cpu can. 19 billion bit operations per second and pin-to-pin latency of 9 nanoseconds
so my summary is that there's a lot of that kind of embedded work going on, maybe more than ever, and you can do things today that were impossible only a few years ago
on the other hand, something like a 32-bit multiplication or a floating-point subtraction is going to cost a lot of instructions, if you can afford it at all