The problem is, from the status quo we have today it's hard to see how we'd get to a point of security without sacrificing general purpose computing.
After all, it's either possible for the user to promote an application to equal the power of the OS, or it isn't.
Oh, sure, you could make it mandatory to ask permission before getting that power. But many security people think we need a system that's secure even in the face of a gullible user who will click through any warning. And there are thousands of applications that enjoy unlimited permissions today; for them, asking for maximum permissions will be the path of least resistance.
The alternative is the iPhone approach: Every app in a sandbox, and no way out of the sandbox even with the user's consent.