Class A amplifiers

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.

Yes.  Don't believe everything you see on the internet.  Most of it is rubbish.

I bought my 1987 Krell KRS200s in the early 90s.  A capacitor blew in 2007 (after 30 years) and I had them refurbished, mainly all new caps.
Yes they do run very hot and in due course components will suffer.  But they are running fine currently (pun intended), so on this amp refurbs come at possibly 25-30 year intervals, not 4 year.

Of course if the manufacturer puts in cheap or sub-standard components then the period to breakdown could be much shorter.

As I say to the Class D community: you get what you pay for.

Vintage amps are cool.  I listened to my first Levinson ML9 (100 wpc, Class A) connected to a monster sized pair of Apogee flat panel speakers.  Woooweee, that amp and those speakers were a match made in heaven.  The thing about though is that an Apogee flat panel speaker is "voiced" with an emphasis on midrange; this means a dry sounding amp would be perfectly matched to an Apogee because the neutral sounding amp plus a rich voiced speaker evens out.  

Back to vintage Class A amps, if you are looking to buy a Krell KMA, KSA, FPB or any sort of these Krells, all these amps are biased very high so they get HOT, like hot enough to cook an egg.  When it comes to Levinson, a ML anything will need service because the capacitors are old, and old technology caps are not built as well as new Nichicons, Black Gate, Mundorf, etc. capacitors.  If you absolutely have to buy a vintage Krell, Levinson, or other brand, call George Meyer AV in Los Angeles.  They do tons of this capacitor replacement work, and they can tell you about specific amps and give you worse case scenario pricing to bring say an old amp back to life.  The only Levinson I would personally get is a set of monoblocks, as in the 20.6.  Everything else, meh, IMHO - hah!  As for old Krell, I think the KSA50 and KSA100 are fairly musical, but for the money you spend to acquire and fix it, hell you can get something much better.