I find it hard to understand the resistance to use of RTOSs.
Well, there's one set of people who doesn't understand what an RTOS is, what it would give them, or how they'd choose between multiple options. They have enough problems figuring out how to divide a program into functions, much less into concurrent "tasks."
There's another set that understands, but is worried about the complexities that you know or suspect comes with it. Perhaps they've been burnt by a negative experience with an existing RTOS. Or they're worried that they don't want to increase latency to get certainty. Or they're just comfortable, given the size of Arduino, that they can get along without it. Case in point:
CERN has used LynxOS in control systems for over twenty years.
Some of my colleagues left the lab to develop VxWorks.
So given 20 years of experience with an RTOS, your coworkers were so frustrated with it that they went to work on a different RTOS ? 
A RTOS is not like learning a new programming language.
The hell it isn't. Especially if the product is already intentionally blurring the lines between "language", "library", and "run time environment."