Arduino was the first (and then, the only) simple microcontroller, that you could give to someone, even a child, give them a 5 page tutorial, and they'd be able to blink leds, capture a button press, a potentiometer input and display the results via leds or 7 segment displays. Every other microcontroller needed a special board to program it, you needed a thick book to program it, take care of registers, use some weird windows-only ide and usually none of that was free... arduino was just different, one board, one cable, free software and good documentation.

Of course, an ecosystem has grown around that, so you could attach pretty much any popular device (sensor, display, input device,...) to it, and the library was already made for you to simply use it in your project.

But now, years later? There are many such controllers, from esp8266/esp32 to rpi pico, with additional features (wifi included) on better boards (displays, buttons, interfaces,...), for even less money.

Yes, the original arduino was important.. but if it vanishes overnight, very few people would notice.