Maybe two different things here: SBCs that run Linux versus microcontrollers (MCUs).

MCUs are lower power, have less overhead, and can perform hard real-time tasks. Most of what Arduino focuses on are MCUs. The equivalent is the Raspberry Pi Pico.

In my experience, the key thing is the library ecosystem for the C++ runtime environment. There are a large number of Arduino and third-party high-level libraries provided through their package management system that make it really easy to use sensors and other hardware without needing to write intermediate level code that uses SPI or I2C. And it all integrates and works together. The Pico C/C++ SDK is lower level and doesn’t have a good library / package management story, so you have to read vendor data sheets to figure out how to communicate with hardware and then write your own libraries.

It’s much more common for less experienced users to use MicroPython. It has a package management and library ecosystem. But it’s also harder to write anything of any complexity that fits within the small RAM available without calling gc.collect() in every other line.

Yes. One looming concern here is that if the new Arduino is happy locking stuff down, the Arduino IDE story could end up being murkier like the PlatformIO story.