They have been cheap for many years. There are so many great cheap options out there now with good support, ESP, ESP32, Raspberry Pi Pico, Pico W, WeAct Black Pill (STM32), various nRF2 boards.

At this point Arduino is just extremely overpriced and probably only still relevant because it's a household name and people want pin compatibility with the Uno and V5 tolerance for reasons.

Cheap boards aren't new, but cheap boards that have a sane development environment and decent documentation are far more novel.

Many of the Arduino libraries have been modified to work with most of the cheap boards (already listed by another user).

If the Arduino IDE becomes a pain point I'm sure an open IDE (or VS Code plugin?) will step in. (There may already be such a thing?)

Edit: another comment suggests PlatformIO + VS Code allows you to avoid the Arduino IDE.

> another comment suggests PlatformIO + VS Code allows you to avoid the Arduino IDE.

s/allows you to avoid/is highly preferable to/ :-)