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

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. 

Time changes things sometimes. My main concerns with used equipment (solid state) are lateral mosfet outputs and with ARC potted "magic electronic component" components... The magic is you don't know what's in there, if it fails you're at the mercy of ARC. Idk if there is a solution for TO3 packaged lateral mosfet outputs as of this date, tons of exceptional equipment could be "bricked" if these fail. 

Most tube designs can be repaired with transformer replacement being the biggest concern.