A short summary at the start would be helpful, otherwise this is a wall of text that isn't inviting.
Great info, just too much at once. For example, I've tried most of these, so I don't feel like digging through all this to figure out if you found the same things.
We don't even know at the beginning how many, or which ones you tried, or what high level requirements you worked with.
They addressed a single-user OS at the time. Abstraction like this at the time worked. It wasn't like these machines were expected to be able to do more.
Legacy stuff just kind of sucks.
I'd say we probably have 86DOS to thank for it.