Compiling code is the easy part, peripheral drivers which implement both the basic Arduino functions (digitalWrite and so on and so forth) and more advanced stuff like SPI/I2C, WiFi, etc. are necessary to make a working "Arduino ecosystem."

I think the reason that ESP was so successful is they provided both a working Arduino standard library so that sketches could just be copy-pasted from other platforms, and a reasonably decent real RTOS-backed development environment (ESP-IDF) for once users outgrew the Arduino training wheels.