What you need RIGHT NOW might not be what you'll need five years from now. I just completed a massive upgrade on my parent's 2013 21.5" iMac a few months ago. Replaced the spinning drive with a Samsung 2.5" SSD for their media files, installed a 256GB SSD blade to run the OS an apps from and doubled the RAM to 16GB. What was a "slow" almost 7 year old machine prior now has new life and frankly comes insultingly close to the 2019 model in real world use. They don't need the 4K res screen as they are in their 70's, and they aren't playing PUBG on the thing so the GPU isn't an issue either. Basically I added another five years or so of life to their main computer for about 250 bucks and an hour of labor. But please, tell us again how "it's pointless to do piecemeal upgrades." Forced Obsolescence is a thing, and Apple doesn't even deny it. Ask any Apple Genius and they will tell you it's more "affordable" to just buy a new computer if yours breaks, and it absolutely does not have to be that way for any reason other than Apple's need to control their silicone and protect their already inflated profit margins. The truly tragic part of all this is it is consumers like you that let Apple get away with it.
I’m still using, although less each day, my early 2013 21.5” iMac with Fusion drive that has performed without issue as one of my work machines. I understand the need to upgrade, if your parent’s iMac came with the pitiful HDD, but for most people, getting 7 years out of a computer is an exceptionally good investment and there is often a “want” in getting a newer device, not simply a need.
I would assume, even for people buying today’s new iMac 27” with Intel inside, that they’ll get the same ~7 years of use out of them, and by then there will have been several other changes / updates that won’t be possible to do via upgrading hardware piecemeal, that will make it an easy decision (or the only decision) to buy the newest Apple iMac or whatever it may be called then. And it won’t just be Apple that is making their computers less able to be upgraded. Every PC manufacturer is moving this direction too, for several reasons. One is that the market for DIY upgrading of computers shrinks every year.
The other is that as tech evolves, in order for it to move forward, you can’t design everything to be replaceable / upgradable. If you did, the upgrades would only be possible using the input type that was from the past, which almost always is going to be slower and with less bandwidth than what we have today (or in the future). If computer companies stuck with this thinking going back to when they were using serial connections, imagine how terrible computers today would be.
Recent advancements with USB will buy some time for more recent devices, given the speed and bandwidth can be enough to run system software on external drives or to run external GPU devices. But even USB-C is changing, as its speed and bandwidth improves, and you will not be able to [easily or possibly at all] upgrade to newer faster USB-C I/O controllers 5 years from now, if they even remain as the dominant connection format.
Just think how people used to complain because Apple didn’t include the ability to add additional storage to their iPhones, while most other cell phone manufacturers offered external sd drive compatibility. But today, in order to have the fastest memory, everyone is using internal only storage and the complaints are all but silenced. Because the reality was, using those sd cards, especially back when they were REALLY slow, made them painful to use. They’ve made improvements to the speed of the sd cards themselves and somewhat to the drives, but they’re pitifully slow, like your parents hdd in their iMac was, when compared to the internal storage out there now.
