Linux

There's stuff for sale like that on the ARM side, but you'll pay out the nose for it, as it's all enterprise-y server-y stuff. For example, you can buy Ampere chips and boards that have ram slots and pci-e slots and all that jazz, but it costs way too much to make any sort of sense at the consumer level.

But yeah, on the consumer side we're never going to get modifiable and upgradable systems from the current crop of SOC vendors, and, worse, the x86 duopoly looks to want to head down the integrated RAM and non-upgradable path too :(