The term is "counterfactuals."

A huge number of philosophers active in the past century (and this century) have talked about counterfactuals, with that talk initially growing out of examining logical conditionals, which have been formalized since Aristotle, and they have an important connection to modal logic, which is a very popular topic. "Possible world" talk is very popular too. Possible world talk deals with counterfactuals/modal logic.

Counterfactuals don't need "defending"--it's a very natural way to think. People are always saying things like, "If only I hadn't made that decision," etc.--that's a counterfactual.

The challenging aspects of counterfactuals are not their occurrence but things like their truth conditions, the semantics of them, etc.

There's a thread active on the board about Saul Kripke's book Naming and Necessity. That gets into a lot of issues about counterfactuals.