It’s obsolete, until it’s not.
You can’t prove a negative.
The absence of another world war in Europe in the last 70 years is not proof, but it’s strong evidence that NATO served it’s purpose.
There is no way to say what would have happened without it, and there is also no sure way to assume it will prevent war in the future, but all other things being equal, why would you want to find out?
I’d rather walk softly and carry a big stick, than to play lame duck and see if any foxes pounce.
NATO was repurposed to become Team America World Police.
Now it’s just World Police State.