Great, but take the example in the article. I roll in with my heavies and at gun point force enough of you to vote that the contract was complete even if it wasn't. You can't engineer people away and when you try you've undoubtedly missed something.
Smart contacts exists so that you don't need the heavies nor the guns, you can just exploit a "bug" in the contract and run off with the cash, preferably without anyone knowing who you even are.
It's nice when everyone abides by the same ruleset. But there will always be individuals that will behave outside of it.
Like, we can all play capitalism and try to outcompete each other, but eventually there's going to be someone that just abandons that ruleset and murders all competitors.
Smart contracts are just a better ruleset, still not enough.
I can still just seize the factory, ledger be damned.
Or deploy a military to seize your private keys. Unless you're sole copy is in your own head it's on a hard drive SOMEWHERE