amplifier longevity

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

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.

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.

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 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.

The product is durable or not, largely  depends on the company, and also depends on the right usage. 

Tube amp could last very long time, and SS amp could fail in short period of time.It is all case by case,and  decided by the points I memtioned above.

I am not sure if OP worry about the complex of usage of tube amp in daily life. If that is you intend to tell,I agree. Tube amp usually ask you to take care more than SS amp in the time being.

In my experiences over the decades, I have had some of both, SS and tube equipment fail. Most have been reliable. I had an instance or two of bad design that eventually led to failure, and bad parts or assembly. 
Some manufacturers are known for reliability and great sound. I won’t buy from a certain manufacturer that has good sound,  but has proven unreliable and problematic.. 

Tube amps can be very reliable as much of the hardworking parts are the tubes themselves. SS has come a long way and I don’t have to have tubes anymore .. 

Assuming high quality like those 2 makers, no difference would be my answer.

Good Tube stuff is designed to handle heat, so there isn’t a lifespan issue that I have experienced. My quality SS equipment has been very reliable, it can get hot also.

My reliability issues have been with the weakest link, usually a moving part, look for them: a speaker terminal or push button breaks, a jack panel loosens, ground connection screw strips, loosens some how.

When I buy cameras, it’s also the weakest link. I try to anticipate, avoid problems, a cheap plastic battery door, waiting for me to break it. If I buy cheap stuff, or good stuff with cheap parts, I treat it’s cheap moving parts more carefully,

If one had ceramic tube sockets, the other plastic tube sockets, that's a clue.

And, modern equipment, too much complexity, too much software, i.e. for some reasons, an HDMI jack stops working, a ...

Not counting my first tube amp, a VTL.....I have had excellent reliability with tube gear.  My current amps are 8 years old , look and work perfect still

carver crimson 350 monos.

amazing longevity and reliability!

Bryson has issues, not often!

odyssey audio has been reliable.

McIntosh is reliable !

Ideally in theory…
But I have had both varieties fail on me.