The narrative will no longer be "Why did we get in bed with the Russians for our energy?" and instead will be "Too bad about the broken pipeline!"
Pipeline B of Nord Stream 2 is unscratched and can deliver 30 billion cubic meter of gas per year.
Gazprom now says that two of three "destroyed" pipelines are now in a stable condition hinting that they can be repaired.
Why would the US only destroy 1/4 of the pipelines?
Now your turn. Who other than the US benefits from this?
If the plan is to come in later as the savior of an impoverished Europe then yes, he does need to blow it up. Shutting off the valves makes him the bad guy, and people will remember. Having it blown up by some unknown party, ideally the US, lets him pretend it was all out of his control and he's doing the best he can to help his dear friends in Europe.
Nothing in the world could make him not look like the bad guy in the west. Shutting/not shutting valves would change zero opinions of Putin.