Where does Apple cross the line from being able to sell a completely closed garden product to having to allow fair competition? If iPhone had no App store and

Where does Apple cross the line from being able to sell a completely closed garden product to having to allow fair competition? If iPhone had no App store and only had official Apple apps we wouldn't be having this conversation, regardless of how popular iPhones are. I think Apple is in the wrong, but a critical issue what threshold or criteria must be met for them to be required to offer an open(ish) platform? *open(ish) meaning allowing alternative app stores or charging industry standard payment processing fees.