Isn't it odd that a 8 year old Mac is still perfectly fine, but every one that is still being sold is hopelessly antiquated?
I'm not sure what you mean. Is it just a complaint that Apple doesn't always update their hardware often? By your own logic, that complaint makes no sense. If the 8 year old Max is perfectly fine, then the "antiquated" Mac that still has last year's technology should still be perfectly fine.
Here's something that's worth understanding: You can generally tell how long a hardware vendor expects you to keep their equipment in service by how long their longest available warranty is. For most Apple hardware, the longest warranty available is 3 years. For mobile devices, it's 2. Dell's default warranty, for example, is also 3 years, but they'll sell you an upgrade for 5. Dell's signaling that they expect you to get a new computer every 3-5 years, while Apple is signaling that you should be upgrading every 3 years or so.
Now obviously you don't *have to* upgrade that often. Apple still supports older devices with their software releases, but obviously certain kinds of support start dying off after that time. The first thing that happens is that the warranty is over, so they won't fix it for free. After that, they may fix it for an extra fee, but eventually that goes away, and they simply refuse to even try to fix it. That often happens around the time they stop manufacturing replacement parts.
But eventually, everyone discontinues support for everything. If you can get Windows 10 installed on your 8 year old Dell workstation, Dell isn't going to stop you. At the same time, Dell isn't going to go through any trouble to help you do it. It's the same thing.