What's labor intensive about it? Mass producing and assembling PCBs is very cheap. A Chinese manufacturer will make you an injection mold for $2000 and then you can produce thousands of plastic parts for almost nothing.
I think the mistake you are making is that the processes that make 1000 units are completely different to making one unit ten times. The design gets streamlined for manufacturing. Nobody is going to hand solder wires to fix a botched PCB layout 1000 times. You'll just redesign the PCB layout and try again.
Did you miss something? This entire thread is PRECISELY about NOT using mass production capabilities, as in a hobbyist builds a few of them. My point earlier was that it can be profitable if you do use mass production facilities.
I think the mistake you are making is that the processes that make 1000 units are completely different to making one unit ten times. The design gets streamlined for manufacturing. Nobody is going to hand solder wires to fix a botched PCB layout 1000 times. You'll just redesign the PCB layout and try again.