Great explanation, thank you.

> This comes at a cost of making adding and removing components from an entity more expensive.

I think you could write a design-time tool that takes a simple description file (with hints) and outputs code that stores your entities and components efficiently.

Description file:

    {
      "archetypes": [
        {
          "components": [ "A", "B", "C" ]
        },
        {
          "components": [ "B" ]
        },
        {
          "components": [ "B", "C" ]
        }
      ]
    }

Output:

    class ArchetypeABC {
      A a;
      B b;
      C c;
    }

    class ArchetypeB {
      B b;
    }

    class ArchetypeBC {
      B b;
      C c;
    }

    class EntityStore {
      ArchetypeABC[] entitiesABC;
      ArchetypeB[] entitiesB;
      ArchetypeBC[] entitiesB;
    }