Lifespan of amps and preamps?

The time to replace is when one of them expires and cannot be mended.

if you like what you have, and can't afford to upgrade or refurbish...but sounds like you're still curious...you might be surprised at how good some new entry level gear can sound...and gear with dried out, dead caps, can still "play" for years...

I think the life span of your equipment is different than the utilization of your equipment providing you joy.

I like your equipment and technology has moved forward significantly not only in sources (streamers, DACs and such) but also the cost per sound reproduction in components such as speakers.

I know someone who moved from a vintage system with Carver CD player and and McIntosh pre-amp/amp with Martin Logan speakers tied to structured wiring providing music through in room speakers in the kitchen, patio and garage move to a Sonos set up because was easier to use at a price point versus upgrading a the multi-zone audio system in the house.

Define your goals and budget then you may get some ideas on what you can add and if there's a secondary market for what you may not be utilizing.

There are two questions here, I think:
1. are they sonically obsolete? 
--> in my opinion, no.  There's lots of great 30 year old stuff, and most importantly you said you like it. QED.--> there may be better stuff, but i'd go very cautiously
2. Can it be repaired? 
--> with solid state gear, 95% of the parts will last 50-100 years--> however, a few very important capacitors are likely near their limit. If and when they fail, it could be catastrophic.--> yet, even with a major melt down, it probably wont kill the rest of the amp. The biggest casualty might be speakers which could see DC or transients.  Are your speakers fused?  I really do advise fuses.....about 1 1/2A fast blow.  Amazing how LITTLE they degraded the sound.  After ripping them out, holder, wiring and all, i put them back in (as in line adapters on my speaker wires)
-> re-capping is not rocket science, but it is laborious. Maybe not worth it.--> while not catastrophic, old power supply caps in particular will slowly degrade the sound. Amazing how good some 30 year old products sounded when i oversaw the re-capping of a dozen or so of them with new, better parts.
Adcom is probably pretty easy to re-cap, relatively.

I agree that you really should look at the modern world of HD streaming, and a good DAC. Need not cost all that much for the DAC there is one i think is a steal at $99 and made right here in the USA.  Got a laptop? With a DAC you have a streamer then.  And computer streamed FLAC/ALAC files sound consistently better than the same thigns fro a CD - not as one might guess, worse.  There are  good tech reasons for this. (of course it must be done right, beyond this discussion).
One more story. I had to clean out my hosue to do a rebuild. I pulled out an original NAD 3020, spica TC-50is, and an AR turntable/SME arm/grado cart.  Carefully inspected everything and got it working - really nice sound from a little system. SO we dont need new, and we dont always need fancy to enjoy music that sounds WAY better than most mass market crap.
Enjoy,
G

Well, these components have provided joy for 30 years. That's spanning the gulf war, the dotcom bust, the great recession to the current global pandemic. That's impressive. If you updated all of these components now, that should get you to age 80. Then, time to update again......