Would there be competition though? Take the mobile app stores: consumers are going to demand that prices are consistent across storefronts (or preferentially buy from the cheapest option), so that market will inevitably race to the bottom. Is that better for consumers than Apple and Google operating them and demanding a high enough margin to support moderation and review? Or hardware; if Apple was mandated to support Android on their devices, who do you think is going to pay for the additional testing burden? Apple's certainly not going to take it out of their profit margin, they're going to forward the expense down the line, as will everyone else.

You'd be right that there'd be a lot of new ideas floated and a lot of competition, but it's hard to see how that plays out in a way that benefits the consumer and make smart phones a better value. I'm no fan of the duopoly, but it's hard to imagine how forcing them to break their services apart winds up anywhere other than the services becoming noticeably shittier and only like 5% of people bothering to change from the defaults anyway. I also think that the innovation around smartphones has all but died, and if there was some killer innovation to be had, it would probably be worth much more in this market than an open market. The market is harder to make distinctions in than even electric cars are, and a distinguishing innovation would surely trigger a bidding war between Apple and Google at this point.

It's a rough case of what I want philosophically (break them up with great prejudice) and what I think is best for the user experience itself being directly at odds.