Class A amplifiers

If properly ventilated, class A amplifiers will live a very long and healthy life. Mo ventilation, Mo better. 

Not true!
I owned a Threshold 4000 for 30 y. Replaced caps once….
total quality

That is a giant load of dog doo.  I have 4 class A amps in house. Two First Watt amps, and a pair of Monarchy Audio SM70mkII's (I run these as mono blocks)  The FW J2 is the newest,built in 2017. I am the second owner of FW F5,that I've had for at least 6 years. Not sure when Mr.Pass built that one. The Monarchy's are at least 10-12 years old. The pair I have I believe are one of the first MKII's that were built. Not a hiccup out of any of these amps. Ever.  Ya gotta love YouTube. 

It depends on the design of the amplifier,whether the manufacturer pushes it close to the limit or not, and whether it is provided ample heat sinking or trying to save on an undersized heat sink.
My good friend has the legendary Mark levinson class A monoblocks (the ones that weigh a ton). He had them when ML came out with tit, and since then has them CONSTANTLY ON. Yup, never power off, and they are happy campers.
With most amp brands I would be worried, but ML class A ere built to last. With that being said, they can probably use re-capping to sound at their best. Ask how much use did they see. If they were powered on regularly, then the caps are in a much better shape vs amp being off for long periods.

No. There are many Sugden class A amps and integrateds going strong 20 to 30 years later....take the A21 for instance.