As Arkless mentioned earlier, the limiting factor for longevity is always going to be parts-availability - even through-hole parts are slowly disappearing, never mind ICs, transistors, lasers, &c.
Perhaps the single item whose longevity (and overall great build) surprised me most was a water-damaged late-60s Teleton (one of Panasonic's erstwhile European brands) integrated I found in a junk shop masquerading as an antiques emporium - one of those brick-shaped jobs from before everything became ~400mm wide. Its wooden case had that 'been-in-a-derelict-garage-for-50-years' look, but the fascia was spotless, and the rear panel only very slightly corroded, so I handed-over a £5er and took it home...
When I opened it up, the interior was absolutely mint, every switch and pot functioned perfectly, and the various electrolytics that I pulled to check were all 100% despite being >50 years old and probably unused for the last 40 of them! So I returned the pulls and left the rest in place, just retouched the solder joints, which was probably pointless, but did drive out a ton of nasty residue...
Cleaned it up, and a mate stripped, rubbed-down and Danish-oiled the case. It worked flawlessly on switch-on: no thumps, no warming-up, just a very pleasant, easy-going, forgiving sound. Can't imagine it was putting out much more than 15WPC, but it drove whatever I wired it to with ease, and barely even got warm, despite hanging its transistors out in the open air round the back.
Foolishly loaned it to a "friend" - never seen it again...