If consumers were equally tied to Walmart as iPhone users are tired to the Apple App store, your argument might make sense. Perhaps if we reach a point where, as a condition of buying your house in a certain city, you are only allowed to shop at Walmart, then maybe it would be close to the same thing. Of course, if that kind of thing were to exist in the real world, people would absolutely expect Walmart to be required to carry competing brands. This of course is a pretty silly exercise because the parallel you are trying to point to doesn't actually exist anywhere in the world.