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Isn't a saturation point a synonym for a plateau?

Once something is saturated with X, you cannot add any more X to it.

A plateau is more like a flat spot before more growth. Saturation means no more room for growth.

My sense of plateau as a colloquialism is that it implies a flattened peak before a descent. Elvis plateau'd in the 50s.

The dictionary's sense of Plateau:

a level of attainment or achievement [1]

What you're describing is, as you've said, a peak. Sure, Elvis peaked in the 50's, then it was a steady decline (not a plateau) since then.