The FTC is free to make the arguments you and others are making here. I would bet you large sums of money that those arguments will not prevail. In fact I'm quite sure they won't bring cases using those arguments because they know that and don't want to be embarrassed. They will probably bring different arguments and also lobby for other regulatory changes, and I'm all for all of that, but they won't come to court saying "Amazon has a monopoly on e-commerce", because lawyers don't like it when judges laugh at them.