The silo parks in USA are still nuke-proof. Because they are far enough apart that you can't get more than one per hit, and it has to be a low or groundburst, but close enough together that the first hit causes a cloud of dust that the subsequent missiles would have to go through to hit their targets. And a nuclear warhead in re-entry has no chance of surviving hitting that dust cloud.
This means that the silos are protecting each other -- you'd need to wait hours between hits to be able to land nukes on all of them. (Or hit them all within the minute.)
So why the need for launch on warning? If you are confident that your missiles are going to be safe from a first strike you could sit there and take the damage.
What you describe sounds more like Dense Pack - which I don't think was ever implemented:
This means that the silos are protecting each other -- you'd need to wait hours between hits to be able to land nukes on all of them. (Or hit them all within the minute.)