New Mac mini has a slotted & removable SSD — but don't expect upgrades

The ability to upgrade a Mac would definitely be useful. I understand that this is a cash cow for Apple since it means people have to choose the model that will meant not only their current needs but possibly their future needs too. 
In many ways it would be good to buy a base model and overtime upgrade the memory and SSD if that is all you can afford at the time.  I've certainly done that in the past for Mac desktops, minis and even MBPs and its extended the usability of the device for several more years.
With Macs now being extremely powerful, the CPUs are able to handle OS upgrades for years.  There used to be a time where a new OS upgrade would significantly slow down a machine.  This M1 MBP that I got in 2021 is still speedy and will last several more years.  I purposely got a model with 512GB SSD because I know that 256 can be very limiting.  I also got 16MB of RAM which though less important for me, 8GB could become a bottleneck.