But the fundamental difference here is that most people buying SBCs are treating them as self-contained COTS compute modules.

As someone that has worked with these EVKs for decades, you never treated them as part of your final product. But with the advent of Beagle and RPI, that's where we are now.

But when you buy an RPi or Beaglebone as the core of your industrial smelter and expect support for that module comparable to, say, a module from Kontron? You're going to be disappointed.