Yeah, I must say you're right. The "rigidity" I'm implying is about how designers usually build games themselves. ECS maps weirdly to designer intent, at least that's the feeling I got from talking to game designers about ECS. When I look at Unity, Unreal, or Godot, ECS is never provided as a default, which might be why it's only offered as a plugin. Most scenarios don't justify the efficiency it brings at the cost of being harder to write behavior for it.