Yes, evaporation can cause a slow degradation of a battery. Usually, when a battery is degraded, one can notice that the engine does not turn over as quickly as it used to.Has nothing to do with the plastic. Heat can cause batteries fluids to evaporate and the internal components corrode.
But most batteries that fail because of heat do so rather quickly (sometimes very suddenly without any warning), and because of structural internal failure. That is why AGM batteries were developed, because they hold the acid in Absorbed Glass Mats (instead of flooded cells), so the acid cannot leak into an adjacent cell and neutralize the chemical reaction in the battery that holds the charge.
So in conclusion, I strongly disagree with your statements.