The RPi itself is still going strong.
Reasons to prefer a pi over a mini PC - easy access to GPIO pins; small(er) form factor; power efficiency; lower weight; still cost if you can work with zeros.
Reasons to prefer a mini PC over a Pi - price to performance ratio is often far, far better; size is "good enough" for people just after a small computer (rather than an electronics project, POC, etc); you are, in fact, reducing ewaste.
In short, if you just need something to run Home Assistant or Plex on ProxMox or similar, you would find more reward in a mini PC than a Pi, particularly in performance.
The bias at play is that people see "old" and equate it with "bad performance". That heuristic only works when comparing like for like - yesterday's mini-PC to today's mini-PC.
I might be hopelessly biased since you would regard me as someone who keeps using computers when they are "past the point of uselessness," which is not a phrase I'd actually use.
A lot of the stuff I do to keep computers running after a long period of ownership does not even apply to the Pi, which is nice. I won't ever have to replace the fans on one of my Pis (the case design of the prototype Pi 5s we've seen notwithstanding), but that's a standard thing on PCs. There's maybe one capacitor on a Pi's board? I've never needed to think about it. I've guiltily thrown away and replaced soft power supplies on PCs, even though I had a friend once who knew how to fix them and I should really learn to do the same, but there's considerably less guilt when it's a USB wall wart. I've pitched a few SD cards belonging to Pis over the years (never the proper SSDs, though) but that has never been nearly as painful as a drive failure in a PC.
So I guess I'm saying I would factor in the maintenance burden.
> The bias at play is that people see "old" and equate it with "bad performance".
I see "used electronics" and "eBay" and think of "wasted time," "frustration," "incompetence," "shamelessness," and "fraud."
The other thing I would factor in is that my time is worth something and eBay's main goal, when it comes to used electronics, is to waste it.
> In short, if you just need something to run Home Assistant or Plex on ProxMox or similar, you would find more reward in a mini PC than a Pi, particularly in performance.
I'd split the difference here and say I'd be happy to run Home Assistant on a Pi and Plex, quite possibly, on a PC (but I'd want to at least test on a Pi). I'd be happy to buy a new PC for the purpose from a trusted retailer, or build it myself.
Let's put it this way: you pop in your SD card, set up a username/password, a few other settings, and you end up with a system that already has Geany and Thonny installed. Go to Help -> Bookshelf and find all the issues of MagPi and Hackspace for inspiration and learning, and a couple dozen books like "Essentials - Code Music with Sonic Pi" or "Get Started with MicroPython on Raspberry Pi Pico". Search "Raspberry Pi Hats" and get a lifetime worth of reading and learning there, hitting just about any domain you could care about.
The vision of the Raspberry Pi wasn't to create a cheap, tiny PC, it was to create a learning platform, of which the cheap, tiny PC is the basis. More than any other computing device I've seen, it really lives up to that vision.
Little 1-liter office PCs from HP/Dell/etc. have MSRP closer to $1000 when new, meaning a correspondingly larger BOM cost, and are built to last until obsolete by PC OEMs who have years of experience doing that. Even well used units have better prospects for durability and longevity than new Raspberry Pi boards.
The idea that a machine becomes more reliable as the number of parts increases and the cost increases is an interesting one, though.
Sorry, I stand by my opinion that Pis are hobbyist playthings. I'm sure they were reliable for you but you probably weren't trying to do anything serious with them either.