I think it depends on what hardware you look at.
Eg, early PCs were expensive and generally of good build quality. However, the very early ones had under-specced and unreliable PSUs. Replace the PSU though, and it can last a long time.
In the late 286 era/386/486 era, cheap Taiwanese clone hardware flooded the market. A lot of it was quite buggy and unreliable. But a 'good' 286/386/486 machine from a high-end brand such as IBM or Dell would probably last forever.
I guess in the Pentium era things started to stabilize more. Chipsets became more integrated, which in turn made motherboards simpler, and therefore more reliable.
But then there was the problem with poor caps. In the search of ever cheaper components, many suppliers chose cheap Chinese caps. These were built using a reverse-engineered formula for the chemical component, which was flawed. High-end boards used expensive Japanese caps, which were very reliable. The others were prone to burn out in a few years.
Once that stabilized, RoHS happened... lead was banned from solder. The replacements were not as 'elastic' in nature, causing soldered joints to crack from thermal stress (repeated warming up and cooling down, worsened by aggressive power management).
But that is under control as well now. I think we're in a reasonably reliable era currently.
I also guess that the older the hardware is, the more 'bad apples' have already been weeded out. As in: the bad hardware broke down and was thrown away already. All that has survived has to be pretty reliable by default.