Why do you believe there is no free market on mobile phones? I mean, what exactly forces you to pick an iPhone over anything?
Don't want to sign up to one multinational behemoth? Well, your choice is to sign up to one other multinational behemoth.
I don't think that the manufacturer assertion is true at all. You can go to any random online store and get dozens of brands and manufacturers. It just so happens that popular demand focuses only on a hand full of manufacturers who are outcompeting everyone in the free market.
Take a look even to Android's market share. You have four manufacturers with double digit market shares, and a couple of dozen entriee. Is that what you call a monopoly?
https://www.appbrain.com/stats/top-manufacturers
The OS comment is even more badfling.
https://en.wikipedia.org/wiki/Comparison_of_mobile_operating...
A good way to think about it is this: if Windows was as closed down as iOS, and took a 30% cut on every application purchase, would regulators have intervened?
The secure solution is to treat every app as malicious and put it in a sandbox where it can not cause harm. See also Android and ChromeOS.
So, no, sandboxing everything in sight isn't a useful solution. Your sandbox will just imprison us all.