Personally, I leave my main computers on 24/7/365 as the biggest stress on electrical components is the surge when you power them on. Think when light bulbs usually blow!
I would disagree, while thermal cycling is a potential risk to PCs, most PCs modern should not have an issue with it during its expected life.
There are other risks to PCs life expectancy, e.g. they have fans that can suck in a huge amount of dust, the dust will reduce the thermal cooling efficiency and thus reduce component life. The fans are also mechanical so will wear with usage.
My experience is most trouble comes from cheap PCs and those often fail whether left on 24/7 or not
Nostalgia is memory with the pain removed