This borders on counter factual history but I think we can safely address it.

The extent to which US presence was necessary to hold back the Soviets can be debated endlessly but we can at least point to a few crises wherein it obviously made a significant difference.

Conveniently, these pretty much all happen in Berlin.

The first and second Berlin crises (I know a few historians who like to enumerate beyond those two but we will stick to the non controversial ones) amounted to attempts by the Soviets to either consolidate their control of Berlin or score some kind of foreign policy victory against the US by way of the same.

In the first, a US lead airlift broke the blockade of Berlin. In the second the US broke the Soviet line with the threat of an armored advance.

In both of these scenarios the presence of US military power in Europe made a tangible difference. To that end I suppose it would be "necessary" to hold the status quo as we understand it today, though that is probably not what you meant by "necessary."

Of course we can go down the rabbit hole asked if the crises would ever have happened had the US been absent from the theater in the first place, or if we forstalled a Soviet push to the Atlantic but that's a discussion for another sub.