Much depends on the category of equipment you are looking at. AV kit etc tended to be all linear (and much of it still works today). Computer (as in PC) PSUs of 20 years ago were all SM by then, but that kit probably aged and got retired for other reasons before the PSU became a problem. Although to be fair, I don't recall the '286 / 486 era kit suffering as badly with motherboard failures - although the maximum current demands will have been lower, and heat was far less of an issue.
Computers of 30+ years ago were still in the "home computer" era, and many of those were still using linear supplies and much much lower currents. I sold a early 80s vintage VIC-20 system a while back and that was still working well ;-)
Consumer electronics is a different ball game now, especially with much of the "built to a price" tat coming out of China. LCD monitor / TV backlight inverters seem particularly prone to crapping out after as little as 18 months. Of those I have repaired recently, all bar one had been failed caps in the power PCB. One was was a monitor that had already been recapped, and inverter step up transformer had failed.