As an embedded programmer in my former life, the number of customers that had the capability of running their own firmware, let alone the number that actually would, rapidly approaches zero. Like it or not, what customers bought was an appliance, not a general purpose computer.
(Even if, in some cases, it as just a custom-built SBC running BusyBox, customers still aren't going to go digging through a custom network stack).
The customers don't have to install the firmware themselves, they can have a friend do it or pay a repair shop. You know, just like they can with non-computerized tools that they don't fully understand.
I’m not talking about your buddy’s Android phone, the context was embedded systems with firmware you’re not going to find on xda developers. A “friend” isn’t going to know jack shit about installing firmware on an industrial control.
(Even if, in some cases, it as just a custom-built SBC running BusyBox, customers still aren't going to go digging through a custom network stack).