Amp longevity, if well taken care of --the risks of purchasing used equip.

I purchased my Yamaha RZ-Z9 new in 2003 and it has over 35,000 hours and still plays just like new.

...Maybe the fan is a bit louder.  Never been serviced.

My amp is from 1977, it has worked beautifully. I did do a full referb 2 years ago, it does sound much better after, but it has never let me down. 

If something is going to fail, it will fail, doesn't matter if it's a week old or several decades. 

As a 20 something I bought an Adcom preamp and amp for $300! Adcom was in New Brunswick NJ close to where I lived I took it to them and while I waited went through the entire amp and pre making changes as they went and gave it back to me free of charge!! 

I purchased a McIntosh MC2125 power amp new in 1979 and kept it until 2022, with a single refurb.  It sounded just as good the day I traded it as it did the day I bought it.  I would expect the answer to your question to be very, very brand dependent, based upon the build quality of the product.

I have 7 vintage amps from the 1970’s to 1980’s from Audio Research and Mark Levinson. They are in regular use, they all work well and none have had service.