Lifespan of amps and preamps?

I agree. It takes passion to own vintage gear. As an example, a newly produced Scott 340b volume pot off ebay costs around $100.

To OP, if you want to keep your system, you should at least do a recap, which I think is now a must. And as I said, some other maintenance like cleaning, checking values of resistors and other components. By experience though, electrolytics are almost always the critical issue. 

audite84 is right.  Electrolytic caps decay over time.  Even if the units are working, it's unlikely that they are at peak performance.  A 535 is a very good amp, and well worth having this done.  If you have been happy with the performance, it's probably more economical than buying new.  

Sounds like a nice sweet spot for you. I have an NAD 3155 (bought new in '85) which I still use as a pre. If it works for you, go with it unless it starts to sound bad, you can always renovate. That GFA 535 sounds like a keeper, too. I would say, you certainly got great value out of the system.

Well, let's see.  I am using, for now until I can find a decent used ARC amp, the Hafler 500 I built myself (they used to sell it as a kit) sitting at the desk in my shop in 1975.  Had it tested a month ago.  He adjusted the bias and it measured at spec in all areas, old caps or not, I guess.

Reason for visit to the shop:  Added a newly purchased ARC SP-6 from the late 1970's.  Replaced two parts in that and it measured excellent by the tech, who is actually an audio engineer of high repute with all the right measurement gear.

SO, how old is too old?  I have no idea, but I am very pleased with my system on my Tympani I-C's, which sound like they did 40 years ago.  I have another pair that was completely rebuilt by Magnepan, but they are still in the box until I move.

Would I LIKE new ARC and Maggie stuff?  DUH!  Unfortunately, I am retired now and just can't justify the expense, so while the stuff is definitely old, it plays music that sounds the same as when I plug in my guitar or bass, or play my cornet or drums, which is the standard I personally use. 

Would new stuff improve the overall sound? No doubt--progress has been made in every field.  If you like what you are hearing, go to a GOOD, PROFESSIONAL shop and listen to new gear in your price range.  TAKE IT HOME and plug it into your current stuff and listen.  If you like the new stuff and can afford it, buy it, I guess.

Remember, as everyone here knows, your ROOM is the most important part of any system for reproduction accuracy.  Either way, enjoy the music!

Cheers!

Well, I was in a similar situation until last Fall (October 2019). Bottom line, I ended up upgrading important components and my sound is TONS better; I would encourage you to do the same.  I bought my stereo back in 1992 and did not change anything until recently. I thought it sounded great. Now -- I would never want to go back!  The only two components that stayed are my original KEF 104/2 speakers. What prompted me to do the upgrade was a series of small discoveries. It is an interesting story but for now I'll spare you the details. 
The first upgrade was interconnects (various Audioquest). I still remember my surprise at how much better everything sounded. It is one of these moments when you want to go though multiple favorite recordings to see how they sound now. And I was thinking -- "I had no idea how great these KEFs were able to sound all along and I just didn't know."  And: "It's the first time I am hearing this CD sound so great." Then you also know there's no going back.
So I remember being on vacation last September thinking how I want to go back home and listen to my upgraded stereo. But the real improvement came later. I started talking to a local audio shop (Q Audio in Cambridge, MA) and ended up replacing my Adcom amplifiers with Musical Fidelity M6si. No way I can come back to these Adcoms any more!  This time I spent 7 hours straight, listening to a lot of music to enjoy the difference (soundstage, dynamics, etc.) -- and again -- the same old speakers proved to have had the potential I never realized!
Last change was a new DAC: RME ADI-2 DAC FS. I stream into it from Node 2i (Qobuz and my CDs ripped to FLAC) + I am getting new life from my old (also 1992) Adcom CD player via digital coax cable from the CD to the DAC. So now the system is built around DAC -> balanced XLR -> MF M6si. And now I am at a point when I don't want to upgrade any more :-)
But the amp upgrade from the old Adcom was very much worth it.
I am encouraging you to do the same in spite of the wisdom in continuing to enjoy what you already enjoy. I wish I had done my upgrade sooner.