Price wise the gap to a zero W is rather small. Fun as this sounds paying a little extra for a full nix environment and wifi is a better fit for me. I can see it being useful for low power applications though
A more relevant comparison is ESP32.
This way Code Clubs and CodeDojos can use existing IT infrastructure to get their kids into doing some electronics projects, at a very low price.
OTOH for this kind of board $4 seems like about the right price - RP2040 looks like ~$1 MCU to me.
The biggest thing going for microcontrollers is that they aren't vulnerable to file system corruption if you yank the plug on them, making them extremely useful for IoT devices like smart locks / lighting / air quality monitoring / appliances / electronics designed for use in the field where you don't want to sit around figuring out how to ssh in and type "sudo shutdown -h now" instead of just yanking the plug and get on with your day, which you can do safely with a microcontroller.
(Strictly speaking yes you could design an OS for the Pi Zero W that mounts in read-only mode and avoids this problem but it's a headache)
That said the Pi Pico doesn't look attractive to me; I usually use ESP32s for those projects these days.
For these very small embedded systems a "full nix environment" is a major pain. It is massively overblown and not real-time at all.