That is a natural monopoly and not unlawful. They do not use the store to leverage position in other markets, which is where antitrust comes into play.
Bell telephone had a similar monopoly. They owned the phone lines and wouldn't allow any non-approved equipment connected.
Your view really depends on if you see the phone primarily as a platform for apps or as an "apple phone" with some apps as neat little add-ons.
Antitrust comes about when a company has a high enough market share AND uses this market share to bludgeon any competitions. Apple created a market, they own the method of distribution, and they actively use their position to prevent competition with their own products and stop alternative markets from forming. Much like Bell
I feel like only a non-elected, inner circle within Govt gets to decide what these condition will return today. Even if the whole country is split 50/50, its only their retval that matters.
In ideal society, a legal debate is nothing to be proud of. People's lives/properties depends on it.