The web sure is convenient, but for the actual work I do, I could in fact work on a 30 to 35 year old computer. I mostly code, and occasionally I process words or spread sheets. All things I could do on a DOS machine, or even something like an Apple //e. I'd certainly be fine on an Amiga. I'd be on cloud nine with a NeXTCube. I don't know that I'm willing to go older than early 80s, though. I need my computer to at least handle both uppercase and lowercase.
So arguably we've already built computers that last 40 years. Another decade doesn't seem crazy.
This is also why when we eventually do have a breakthrough in AI or quantum computing, it wont change anything. We'll just use AI to serve ads on Quantum Facebook, or something equally useless. So many web frameworks have come out in the past 10 years and more then ever websites are spammy, bloated, and less intuitive than they were 10 years ago.
I love wikipedia because it's more performant than facebook or youtube, it doesn't track me, it doesn't have anything moving or sliding around the page to increase "engagement" ,, it just gives me info without making me fight for it. I wish every website was wikipedia. I don't need react or angular, I don't 60fps buttons with smooth gradients. I just want my info
Yeah, it's mostly about "last 50 years doing what?".
Someone else mentioned the Voyager space probes. I think there are cars from the 70s and 80s with some embedded computers and some of these are still on the road too. The computer electronics can be made robust if desired. The hard part is if you mean "general purpose" and you want to include purposes of the future that we haven't explored yet.
I've recently powered up some "portable" Toshiba computers from around 1990. Aside from the CMOS clock batteries being dead and resetting to the wrong time, they booted DOS and I was able to use their existing programs to inspect the existing data files, delete things, and run a disc scrubbing utility. The vacuum fluorescent display worked like new, the hard drive still worked, etc.
These would still work for word processing etc. But with their RAM, storage, and IO limitations, they wouldn't work for modern use cases with modern sized media payloads.
I recall my 386-class machine that supported my computer science course work in university. It was able to barely decode a short 320x240 ~10 fps MPEG video demo from a research group. Its entire disk space was only about 80 MB, whereas today I may have bigger files than that on my phone.
You could build some kind of Computer of Theseus that has sensible buses and modular pieces to allow it to be expanded over time to support new use cases. I think they are called "mainframes". But, economics aren't going to make this cheap and competitive enough for consumer use cases.
This is what desktop PCs were for us in recent decades. It's not going to make it 50 years, but we got a lot of mileage out of the various buses and power connectors to allow incremental upgrades of parts. Eventually, you wipe the slate to get rid of some of the most legacy parts, buses, and form factors. Nobody's PC power supply from 1990 was going to support a modern GPU, not to mention the changing power needs of CPUs and mainboards.
So arguably we've already built computers that last 40 years. Another decade doesn't seem crazy.