Charles said:

Suppose one creates an algoritm that diables the ability of the OS to connect to the Internet (well, for maliciously naughty reasons, it can connect to the hacker's devious representation of the Internet, anyway...). If you attempt to download this exploit, then you will be warned. If you try and execute the binary, then UAC will prompt you. Or are you saying you can get around this UAC behavior as part of the very UAC exploit that is the basis of this argument?

C

Take my scenario:
You're browsing a website, Adobe Reader has yet another bug in it, an advert on the site injects code into that process and starts executing as the current user. It then launched calculator escalates Adobe Reader and roots the entire system.

What would happen with UAC on full? While Adobe Reader could cause issues and attempt to inject its self into processes IN CASE they get escalated later, a more realistic scenario is that it would be greatly limited within its scope to cause damage. Simply because luck is required (the user escalates something) and it is a lot harder to write.