For most of the war the North was not off-limits. Certain targets were; airfields, the storage depots in the middle of Hanoi, the Haiphong docks, etc.
Completely useless targets, like road intersections that could be bypassed or repaired overnight, were added in an apparent attempt to just keep the bombs falling.
If the very fact of the bombing did not provoke the Chinese to intervene, then hitting a few important targets, and making it effective for a change, would not have either. This was proven at the end of the war when Nixon lifted all the restrictions for Operation Linebacker II.
McNamara held back as part of his freakish and utterly immoral “communication by bombing” strategy, a mistaken effort to intimidate the North Vietnamese with the threat that these targets would be hit if the NVN didn’t start negotiating in earnest. This was a propaganda trap, since he could not then hit them without provoking cries of “escalation” from the pro-NVN media.
This is dramatized in a scene from THIRTEEN DAYS (2000) where the bespectacled accountant McNamara lectures a square-jawed Naval officer about how modern wars are fought. Micromanaging the US military in the Cuban Missile Crisis was deemed "successful" by the Kennedy Administration and therefore the way they planned to "win" in Vietnam.