But I've seen many old and even ancient computers where only –you guessed it– the (CMOS memory/clock) batteries went bad. Basically all of them go dead after a few decades, exchange them for new ones, and the puppy will boot up again.

For more modern devices: I'm now on my third battery of my private notebook (6 year old HP Probook), third battery on a Galaxy SII (!) which still is OK for random tests and to leave around as a music player even running Android 6.0(!). If I had to send them to a shop to have the (internal, glued down) battery exchanged, I would have stopped using both devices already a few years ago.

But just buying the battery for a few (few 10) bucks is cheap enough to risk the investment in an otherwise paid off machine.

I don't think the same holds true for more involved repairs where a mobile phone made of (a few hundred million…) discrete and easily repairable transistors would be handy ;-).