>Which sounds great, but the way things work now tend to be the exact opposite of that, so there will be no trustable platform to run the untrusted code in.

This is the problem with software progressivism. Some things really should just be what they are, you fix bugs and security issues and you don't constantly add features. Instead everyone is trying to make everything have every feature. Constantly fiddling around in the guts of stuff and constantly adding new bugs and security problems.