What I had in mind was the case wherein an application demanded multiple tasks, not necessarily concurrently operating, but operating on the same day, week or month, and without reprogramming.

All the disadvantages of a "real-time-executive" that you mention are there. Nothing can be done by the OS to improve performance, since it reduces it by consuming time and resources.

I see the OS as a user tool, and it does that poorly because it was written by humans whose assumptions that are not always valid.

I see no real need for any sort of OS in an embedded environment, since the tasks are all well defined, the sequence of them is well defined, and they're small. If your application is too big for your MCU, you don't need an OS to fix it, you need a bigger MCU. Not everyone sees things this way, but it's likely to remain that way despite my beliefs or anyone else's.

Operating systems make for topics of conversation, and provide user interfaces. They do those things fairly well. For all the rest, they get in the way and complicate things. The only application that I can guarantee they'll serve well is starting an argument.

RE