Overwatch has networked gunplay and uses ECS to deal with that hard problem.

Cool. I get why ECS is used there. Hard problem, meet the formalism that I will use to think about you.

A case board is not a hard problem. Using composition or a functional style instead of OOP is not a hard problem, or at least not a hard problem that ECS solves.

I'm not sure that we're talking past each other now, but I did want to at least respond to your point about Overwatch and note that ECS is used to solve an actual, hard problem there.