New old systems in the age of hardware shortages

Recently I asked something on the Fediverse:

Lazyweb, if you were going to put together new DDR4-based desktop (because you already have the RAM and disks), what CPU would you use? Integrated graphics would probably be ideal because my needs are modest and that saves wrangling a GPU.

(Also I'm interested in your motherboard opinions, but the motherboard needs 2x M.2 and 2x to 4x SATA, which makes life harder. And maybe 4K@60Hz DisplayPort output, for integrated graphics)

If I was thinking of building a new desktop under normal circumstances, I would use all modern components (which is to say, current generation CPU, motherboard, RAM, and so on). But RAM is absurdly expensive these days, so building a new DDR5-based system with the same 64 GBytes of RAM that I currently have would cost over a thousand dollars Canadian just for the RAM. The only particularly feasible way to replace such an existing system today is to reuse as many components as possible, which means reusing my DDR4 RAM. In turn, this means that a lot of the rest of the system will be 'old'. By this I don't necessarily mean that it will have been manufactured a while ago (although it may have) but that its features and capabilities will be from a while back.

If you want an AMD CPU for your DDR4-based system, it will have to be an AM4 CPU and motherboard. I'm not sure how old good CPUs are for AM4, but the one you want may be as old as a 2022 CPU (Ryzen 5 5600; other more recent options don't seem to be as well regarded). Intel's 14th generation CPUs ("Raptor Lake") from late 2023 still support DDR4 with compatible motherboards, but at this point you're still looking at things launched two years or more ago, which at one point was an eternity in CPUs.

(It's still somewhat of an eternity in CPUs, especially AMD, because AMD has introduced support for various useful instructions since then. For instance, Go's latest garbage collector would like you to have AVX-512 support. Intel desktop CPUs appear to have no AVX-512 at all, though.)

Beyond CPU performance, older CPUs and often older motherboards also often mean that you have older PCIe standards, fewer PCIe lanes, less high speed USB ports, and so on. You're not going to get the latest PCIe from an older CPU and chipset. Then you may step down in other components as well (like GPUs and NVMe drives), depending on how long you expect to keep them, or opt to keep your current components if those are good enough.

My impression is that such 'new old systems' have usually been a relatively unusual thing in the PC market, and that historically people have upgraded to the current generation. This lead to a steady increase in baseline capabilities over time as you could assume that desktop hardware would age out on a somewhat consistent basis. If people are buying new old systems and keeping old systems outright, that may significantly affect not just the progress of performance but also the diffusion of new features (such as AVX-512 support) into the CPU population.

The other aspect of this is, well, why bother upgrading to a new old system at all, instead of keeping your existing old old system? If your old system works, you may not get much from upgrading to a new old system. If your old system doesn't have enough performance or features, spending money on a new old system may not get you enough of an improvement to remove your problems (although it may mitigate them a bit). New old systems are effectively a temporary bridge and there's a limit to how much people are willing to spend on temporary bridges unless they have to. This also seems likely to slow down both the diffusion of nice new CPU features and the slow increase in general performance that you could assume.

(At work, the current situation has definitely caused us to start retaining machines that we would have discarded in the past, and in fact were planning to discard until quite recently.)

PS: One potentially useful thing you can get out of a new old system like this is access to newer features like PCIe bifurcation or decent UEFI firmware that your current system doesn't support or have.