it makes no sense to me that they became slower with every iteration of the OS
Well, I mean if they really want to force you to get a new Mac they could just drop support. It doesn't sounds a good business strategy to continue support your hardware only to slow it down intentionally and make your life miserable XD. On iPhone they do it because of the degradation of the battery over time which makes sense. Indeed could argue if it should be a user choice or not, but makes sense.
On Mac, I don't think they have ever provided any explanation but I do have a theory.
Do you remember when Canonical was trying to push Ubuntu really hard to desktop and mobile so people can run Ubuntu on all their devices and even use Ubuntu mobile as a desktop PC. Mark Shuttleworth the founder of Canonical talked about it in a live broadcast during the campaign they have been in contact with multiple chip manufacturers like Intel, MediaTek, Broadcom, Nvidia and others if they could provide better support for the Ubuntu Window manager in their drivers and they refused to do it because Ubuntu counted for only 0.3% of the global market share and chip makers were not interested to implement any Ubuntu specific code in their drivers for such a small market. This is why the first (and only) release of the Ubuntu Touch tablet and phone was horrible, freezes, lags and crashes was very common and no manufacturer ever wanted to release an other Ubuntu hardware ever again and Canonical had no choice but to give up this dream because of the chip makers.
Now, it is just speculation but I believe that despite the fact Apple is a leader in the industry, macOS could be in a similar situation with only 5% (or even less) global market share. Chip manufacturers doesn't care, Apple is just an other client like Dell, Sony, Acer, Microsoft and so I can honestly imagine, chip manufacturers just simply ignore how macOS works and Apple can not make them to do a macOS specific job the same way as Canonical wasn't able to make them do an Ubuntu specific job unless you order a custom chip and this is exactly what Apple did.
They have manufactured their own chip and demonstrated the potential of macOS on proper drivers by running Windows and X86 software on M1 through an X86 -> M1 translation layer, much FASTER than on native X86 hardware with their useless drivers. So I honestly believe Apple silicon is going to change everything.
Of course it doesn't help your current situation with the Intel Mac mini, but maybe worth considering to upgrade to an M1 Mac at some point in the future as your daily drive and run Windows and Linux in a VM.
www.macworld.co.uk
