Robustness and long term reliability of Arduino boards and AVR chips

8 bit AVRs have indeed industrial applications.

Most of 8bit chips (avr or otherwise) are indeed for industrial (=consumer goods, controls, automotive, etc.) applications. Think about your washer/dryer, some of your blenders or toasters, cell phones, cars, big machinery, etc.

Atmel is just a tiny player in the overall mcu market - Renesas and Freescale dominate there