US antitrust law isn't designed to break up platform monopolies, period. Which is unfortunate, because the kind of all-owning combinations it's designed around aren't really possible in tech, post-Internet.

Microsoft of the 90s was much-lamented, but in reality they extracted far less value than they could have. They could have easily asked for 30% of all revenue of all Windows software, and developers would have had to pay it.

I'm hesitant to advocate breaking up of platform monopolies or duopolies, but a lesser-version of antitrust law would substantially benefit consumers in both app stores and ISPs. In both cases, the monopoly stems from monopolized access to the end customer, which is what would be beneficial to attack.

IMHO, a sliding scale of mandates for increasingly-open third party access based on market share (broken down into each user market!) would be appropriate.

If you want 90% market share, go for it. But you'd better believe you're going to have to provide (1) customer access to third parties & (2) any work required to enable them, at cost.