In some ways, I feel kind of bad about how modern devices are likely to age. Booting up my old Galaxy Tab S3 for the first time in literally a year and a month, I wanted to replace my old Kindle Fire HDX 7 as a spare clock. In doing so, I couldn't help but think about how much of the device will be rendered useless in a few decades rather than obsoleted.
Software and Networks seal our fates
Software wise I've mostly found the death of old machines to be network centric. Things like being unable to run a modern TLS protocol for HTTPS, or the utter lack of a web browser capable of stripping down the modern web. That's what really kills a computer for most people. In the case of my project to fix up a 30 year old PowerBook, I suspect that finding software to study the system won't be too hard. Thanks to the Old Mac Software Archive. In the case of Android and iOS, I expect folks will simply be humbly screwed. That or websites like APKMirror will become more vital if you want to experience old 'droids in a few decades. Weirdos amongst us who like old computers aside, many people are simply stuck with obsolete equipment for years. Either by choice or need.
Over the past two decades we've seen a significant shift towards applications that rely upon remote services to function. Built on top of that the trend for The Network to be The Center of what we do has probably been building for the last four decades. By the time I first met Windows, and left Tandy DOS behind, it was already common for personal computers to be online via dial up modem, or if you lived in the right neighborhood: a broad banned modem. As a kid, most of my software came from the school supply stores which probably had more 5 1/2 floppies in the late 90s than anyone wants to admit. As an adult, many of us are reliant upon Networks rather than Applications.
Case in point: a large part of why I regard my tablet as my main computer: I do most of my common stuff on it. Surfing the web isn't really a desktop task for me: it's a lean back with a tablet kind of task.
Once applications like the web browser and news apps cease to function, most modern devices won't be so easily revisited. In twenty years, I'm not even sure that Android and iOS will have a means of getting past first power on when they are no longer able to phone home and login. The trend has been that strong for networks to matter more than the devices that use them.
I don't think that you should use an old computer to do all your stuff. It's kind of crazy to expect a decade plus old version of anything to securely sign into diddly squat. But it would be sad for such issues to prevent you from playing with an old piece of hardware. Whether that piece of hardware belongs in a museum or in a landfill.
Hardware ages and becomes brittle
One reason that I ended up choosing a PowerBook is because I don't really know the classic Mac operating system. Another reason is the hardware isn't totally kaput yet. Most of the 386/486 era laptops that were Super Expensive and Super Kool when I was a kid are basically gone, and it's kind of depressing even looking around for ones that are functional. Vintage Macintosh systems are pretty beat up as well, but you can actually find plenty of them, and if you're willing to pay and not to specific in model: can likely acquire one that works out of the box. Last time I looked for comparable PCs, I found myself amazed by just how many PC vendors don't even exist anymore!
I'm not sure how long plastic is meant to last. Pretty sure the result is somewhere between too darn short and holy crap long depending on whether you need it to hold up, or whether you're waiting for it to break down in a landfill. I've always found it kind of impressive how long "Stuff" lasts. Also perhaps depressing if you consider a typical Styrofoam cup will probably outlive us all. Perhaps that's actually a better reason to seek glass and aluminum than embrace plastic: devices fall apart with age and decompose with corrosion.
For one thing: internal batteries. Given enough time just about any battery is likely to swell up, leak, or poof. My old Galaxy S5's true end was when the battery would heat and swell and pop the back cover off. In a more modern device like my Galaxy Tab S3: it'll simply spit the damn thing, probably like an egg going splat. I'm sure the glass of the screen won't survive tremendous battery swells. GOD only knows about leaking inside devices that aren't readily taken apart.
Old ass computers on the flip side, in contemporary definitions of oldness, at least hail from an era where machines were expected to be taken apart. Today increasingly machines are designed to be written off if damaged, or are so hard to take apart that it's better left to a technician than the regular consumer. Being designed to be taken apart, older machines are obviously easier to take apart and put back together again.
Which in some ways makes me glad that most laptops I've owned have removable batteries, both the main and an internal coin cell. Versus "Oh wow, is that keyboard sticking up at a funny angle and cracked the screen? Didn't know a lid could look like that..."; which is what I expect today's svelte laptops to look like if you leave them on a shelf for thirty or forty years.