[highfreq]

It is an interesting questions. My feeling is if-then-else falls some where between invented and discovered, something like mathematics. Once we had discovered the need for conditional control flow, any decent assembly language programmers would quickly discover many patterns for how it could be used. Eventually, these patterns were clarified and codified into languages and more abstract formalism. Like in much mathematics, that abstract formalism was am inevitable discovery once we took as axiom the basic logic of program control flow with conditional jumps.

It is such a basic, clear, and useful formalism, it is hard to imagine it not becoming widespread under some name/syntax.

But if-then-else is only clearly inevitable if you take as axiom the existence of store program computers with control flow. I can much more easily imagine that early on we invented a very different way to do computation as a whole. It seems likely that what we have actually invented is on a path toward a local optima.