Not at all! Many devices can be flashed over USB, for example. Many STM32 devices come with a bootloader factory programmed in a read only area that implement U

Not at all! Many devices can be flashed over USB, for example. Many STM32 devices come with a bootloader factory programmed in a read only area that implement USB DFU. Many user-provided software bootloaders can do the same on many other devices. Including the attiny85 using bit banging! And of course with wireless enabled microcontrollers one can update over the air. Nordic NRF5x over Bluetooth for example, or ESP32 over WiFi.