Basic fold equity question

[muffins]

Hi all,

Just a question about the concept of fold equity. I understand that a push has a lot more expected value than a call since you have fold equity as well as showdown equity, and I understand how to use a calculator to work out a $EV figure, and if this is positive then the push is the correct move. I have seen this called an unexploitable strategy. My question is this: The reason this strategy is unexploitable is that you are including the fold equity in your calculation, and your opponent has no fold equity once you have pushed. But can't your opponent change this by deciding to call every push, reducing your hand value to showdown value alone, where you could be effectively 50/50 to win or lose?