Almost nothing you say about modern preemptive kernels is true.
For one-off hobby projects a RTOS can be a good tool to accomplish more with less code and time.
FreeRTOS can do a context switch on a m328 in about 50 us.
The Large Hadron Collider (LHC) sits in a circular tunnel 27 km in circumference. The tunnel is buried around 50 to 175 m. underground. It straddles the Swiss and French borders on the outskirts of Geneva.The LHC is designed to collide two counter rotating beams of protons or heavy ions. Proton-proton collisions are foreseen at an energy of 7 TeV per beam.
It's not that hard to use FreeRTOS. I downloaded FreeRTOS and did a few edits to make it compatible with Arduino. I made FreeRTOS an Arduino library, AvrFreeRTOS. It took about an hour.
as a general programming model, multithreading is very hard for the majority of programmers, let alone non-programmers.
I would not want Arduino to have an(RT)OS running by default.