Here's how I read the conversation.
> Why can't I create multiple accounts? I want this feature.
> Because iOS is single-user.
It's not circular sorry, I meant to say your answer is stating the question.
I don't mean to offend or start a fight. I just read the conversation similar to "I have X why can't I have Y" "Because you have "X". The "by design" is moot when Apple are the designers. It's not some real world construct or difficult problem to solve, they don't have multi-user because they don't want IOS to have multiuser.