" point of disagreement is planning for multi-user capability. "

Multiuser tends to go hand in hand with other things, like good security though. No doubt a modern single user system could avoid the mistakes of Windows etc. But as you're already going most of the way there, why not just build multiuser in from the start?

I'm sure Mr Torvalds never expected to fire up a VPS and have the option of Linux along side big and professional OSes like GNU and Unix.