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

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.

I don't know that we have even an approximate answer to that yet for class D amps in general.  Both the method of amplification and the power supply design are quite different from traditional SS amplifiers.  The current generation of class D amps for sale mostly use designs from the last 10 years or so.  ICEpower audio amplifier modules became available around 2005.

You might want to contact Bel Canto and ask what they would recommend for long term maintenance of your amps. 

35 years of buying used gear. Exclusively!

And still buying used gear occasionally.