It's not that he ESP8266 is fully a replacement and there are still lots of situations where you would lean towards the Atmel chips in the Arduino or others like PIC and STM. E.g. the ESP8266 is pretty limited on IO, not as tolerant of higher voltages, more heat/power, worse for sensitive real-time applications, inferior low-power performance and so on.
But for a lot uses they are a better fit. The ESP8266 is a right time and right place kind of thing - it has enough IO and wifi to be really useful for the hobby explosion of IoT stuff.
But for a lot uses they are a better fit. The ESP8266 is a right time and right place kind of thing - it has enough IO and wifi to be really useful for the hobby explosion of IoT stuff.