Restoring a vintage piece of gear is not a given. Being an audio forum we are naturally going to see lots of posts from people who are trying to diagnose their old gear. I would think for every person who has a failed component there are 50 components that are just fine.
I have also read of many people who got there gear service and recapped who preffered the sound before the work was done.
Maybe I have been lucky. The only gear I have ever had that didn't work was gear I bought that way so I could repair it. Everything else I bought in working condition and all of it is still working supurbly. Beyond Deoxit/lubricant and a couple minutes to check the DC offset, why would I want to mess with a good thing?
Anyway, here is a partial list of gear I use on a regular basis that has never had a full service/recap:
Heathkit AA-1800 amp
Heathkit AA-1600 amp
Heathkit AJ-1600 tuner
SAE 2400 amp
SAE T-102 Tuner
Carver Mono-block pair
Pioneer RT-707 reel to reel
Soundcraftsman MA-5002 amp
Rotel RA-1412 int amp
In the case of the Rotel, it uses Toshiba output devices that are original and unobtanium. Unless it should fail on it's own, I will not restore anything inside. Even the most competent tech can make a mistake.
As to your question, my own "whatever reason" to not do a full restore is like a doctor's hypocratic oath to do no harm, or in this case, do not risk possible harm. As long as I'm happy with how the gear is performing why would I put it under the knife? It may continue to perform well long after I'm gone.