I had a fancy-ish Panasonic for about 13 years, including a variable-speed magnetron (which is a hell of a neat trick). I recently gave it to a friend when the SO downsized it; I expect it to live at least another 4 years of regular use.
It was neither particularly cheap, nor alarmingly expensive when I bought it. Other than some of the superficial silkscreening being being rubbed off from cleaning, it looks and performs like new with zero maintenance over the years.
Aren't they all like this? I don't think I've ever seen a microwave oven retired simply because it suddenly stopped working.
My grandparents' 1975-ish Amana RadarRange was still working fine when she died a couple of years ago.