Oh there were some around by the mid-'80s like OS-9, had a lot of fun playing with it. Beyond the kernel you only needed to load what you needed and in fact didn't have to load the whole kernel. It was easy to extend and it even had a GUI though it was painfully slow. Never liked Intel architecture with its bizarre (to me) page switching scheme and anything from MS-DOS which comes off as a kludge. Even now you have to reboot to do something as simple as dismount a driver. What little PC code I write is as far from the iron as I can get.
I play with PIC chips for fun and even on the low end people are abandoning assembler for C and licensing code libraries. If you look at the 18xxx instruction set you'll find it was written to favor C, never mind the 16 bit chips. I haven't found a need for a RTOS for my projects though interrupt handlers can get a little complex. Mostly it's just easier to let an ISR run to completion with one or two higher priority interrupts allowed. Context switching overhead can kill you.
The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh
