amplifier longevity

I decided on SS 45 years ago and have not gone back to tubes, too much maintenance. Love the V-fet amps by Yamaha and Sony in particular. I haven't tried any of the new tube amplifiers, but tube preamps are pretty nice. Yamaha, Phase Linear, Sony, SAE power amps are all over 43 years old with original output devices. Capacitors and pre-driver transistors have been changed over the years. Of course, the death diodes in the Sony amplifiers were the first to get replaced.

Modern caps are a lot better than what was out in the 1980s.  If you are buying new right now I'd expect either to last you a liftetime (minus tube life).

If you have vintage gear and can afford to get them recapped today, go ahead and you'll be done. :)

I think they are about the same.  Alot depends on how hot the amplifier runs I think.  I have heard that really hot amps cook the capacitors and make them age faster.

Too many factors involved. The design, the operating mode (Class A vs AB), parts quality. That also doesn't assume catastrophic tube failure, or shorting the speaker terminals of your SS amps.

 Under normal use, no reason either can't be completely reliable.

The exception to all of this is Class D. I am seeing an up tick in Class D failures of amps that are only 3-5 years old. Some have even become non-repairable due to obsolete parts or boards.

Depends on build quality does not matter tubes or ss..