While that's true, RP2040 is $1 these days. Though it requires external ROM to run, its a popular device and many boards (not just RP2040) are under $5. (ESP32 in particular).
So there's a wide number of under $5 boards available that can run MicroPython, providing an easy start to embedded programmers.
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Of course, 8-bits are needed for minimal power consumption and maximum efficiency, with an ability to work off of 128kB of ROM and 8kB of RAM at under $1.
But more importantly, 8-bitters are more complete systems. Better ADCs, often DACs, OpAmps, and other advanced features that RP2040 (and other 32-bit systems) got rid of to make room for RAM or ROM.
That's the thing: 8-bits are more convenient and easier to use not because of code issues, but because they come with all the tools you need on one platform.
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But beginners don't know how to use those tools *anyway* and are instead focused on blinking lights or basic I2C / SPI controls. I think grabbing a 32-bit Cortex M0+ (like RP2030) or similar system to run MicroPython isn't a bad thing.
Its inefficient and better designs exist. But beginners shouldn't focus on optimizing yet. Its just like BasicStamp or BASIC-for-8052 back in the day IMO. You need a beginner platform so that young teenagers can experiment with this stuff, and young teenagers aren't going to read the 300+ page manuals that come with an AVR64DD32 to fully understand an 8-bitter's operation.
But even a teenager can plug in a USB port, click a few buttons on an IDE and load Micropython to a RP2040 / ESP32 / whatever.