The microcontroller landscape is shifting rapidly. I think many of the things you've mentioned - no dynamic tasks, no filesystem, etc - are appropriate for some devices and some software but not all.
MCUs today are pushing the boundary on performance. We'll see GHz Cortex-M7 chips (already 600MHz+ today) running an RTOS in the near future, with external DRAM and flash. Users rightly want to push a LOT of functionality on them and that comes with advanced OS usage, including dynamic memory and threading. The developers using these systems are increasingly coming from an embedded Linux world as the performance line blurs, and POSIX is familiar.