Right, that's my question. You can use flash and display and whatnot without an OS. When are you using a microcontroller that you'd need the overhead of an OS where an SoC wouldn't be appropriate?
Again, these are not binary. I may decide I need a more feature-full (more "OS-like") task switcher for what I'm trying to do, without going to a full OS (by some definition). I may decide that I need some aspects of SoC in my microcontroller. (Youi can get pretty much any set of features with any core; it's more the number of desktop-like features that makes it a SoC.)
To your specific question: I'm not sure it goes that direction, because a full OS depends on the presence of certain features on the chip. I can think of some examples of it going the other way, though - of an SoC where you don't need or want a full OS.
