Linux doesn't scale down very well though. I really don't need the full Linux API for a lightbulb. Most embedded OSs are microkernels for this reason. If any mega-trend has a chance of unseating Linux and generally disrupting the OS space it's IoT.
A lightbulb will have an RTOS, not a microkernel. While microkernels provide utmost isolation between processes and well-defined primitives for communications, a lightbulb might not even have separate supervisor and user modes.
A lightbulb won't even have a RTOS. It will be a state machine or series of them in C language on cheapest MCU imaginable. Probably 8- or 16-bit. That keeps the cost at nickles or dimes.