Most SBCs use socketed eMMC chips available in various capacities. Pine64 and Odroid's eMMCs are compatible, at least, and live on a tiny little board you can swap out.
I don't see the cost and complexity as being worth it, though. Just stuff the things full of RAM, have lower capacity options, and solder it - same as we see today. Yes, it's nice on paper to swap the RAM out, but I guarantee these sockets aren't cheap, and for something that's used once or twice, eh. Not worth the hassle...
I like things that are easily repaired, but I also don't see a huge wave of ARM SBC RAM failures. They're already repairable, it just requires hot air rework and a bit more skill than swapping a DIMM.