It would look really, really bad for the US and NATO to intervene...and they would intervene. It would be the Western Hungary 1956 or Czechoslovakia 1968, even if it does go down to subterfuge, spies and assassinations. The shoe is on the other foot. The clear point, for the Soviets and the world, is that you say you're open to the self determination of free peoples, yet when it runs contrary to your vision, when it goes outside your control and bumps up against the walls of freedoms you allow these people to exist in and yet pretend there are no walls, you stop it. Moreover, you'll let fascists like Franco and dictators rule and take over nations. And if a right wing cabal had taken over France, you would have preferred that to a Communist takeover. Yet, because they are Communist, that is when you crack down? You will allow a tyranny which makes no excuses that it is dictatorship, rather than Communism as asserted by the will of the people? Less freedom in the "Free World"? That is the hypocrisy. And even for anticommunists outside of the United States, it is a blunt statement that the United States is not interested in a free, independent Europe. Just like the Soviets, it is interested in a sphere of influence for global assertion of power, which it just so happened that permitting a free democratic Europe was best for. Again, the concept that the freedom is actually limited, and it was simply wider than the USSR allowed to its satellites, but when the satellites of the United States went outside of those limits, the US reacted in kind.
You can debate the complexities of all of that. And that is not an innate truth and fact concerning what that intervention would be, or the morality of it. But the point is that is the message that is ripe for exploitation and assertion by the world, Communist and otherwise. And in the years that follow, it will have a serious, detrimental, deteriorating effect on the US and West in the Cold War. I think, seeing where the situation in Europe began to evolve by the 1980s, it would make Europeans chafe under the idea that there freedoms are not actually free. It is only that they had existed within confines before 1968, and had not bumped up against those limits. And when they see those limits are there with 1968, they will never unsee them, and view themselves as in a cage of mostly benevolence but a cage nonetheless.