That's actually how all mobile operating systems work right now. You don't get root (admin). The only way to get it is to jailbreak/root, which essentially involves running a privilege escalation exploit or patching the operating system. In any case it's definitely not as simple as "sudo" or "run as administrator". However, even without root, most users manage just fine. They can browse the web, edit documents, play games, and install whatever apps they want. There's no reason why this model can't be extended to desktop operating systems. You can't do it on mac/windows yet because of legacy reasons (eg. needing to install legacy drivers or apps).