>It has dual core if you need it, and the fun little IO coprocessors

I think you're missing the point of what made arduino so popular. It's not the HW itself, it's that you can plug in whatever display, sensor or motor driver out there, and there's ready made templates in the IDE that gets you running immediately, without you having to know anything about how the HW or SW works under the hood.

The lack of dual cores or "fun IO coprocessor" whatever fun is in that context, was never an issue for the arduino.

There's a virtually unlimited number microcontrollers and boards out there for tinkering or production, that are more powerful and have more features, but they all have a higher technical barrier to entry than the standard Arduino out of the box.

I don't wanna have to read datasheets and erratas just to learn how to use a second core, deal with shared memory between cores, or how to configure the GPIO of the "fun IO coprocessor" just to get a LED blinking to work. That's not what fun is to a lot of people. Fun is getting the motor spinning until my coffee finishes brewing and that's where the Arduino ecosystem USP was versus other more powerful platforms.