Sure, but there's a good argument that that should be an end-user issue, and not something that the OS/Phone manufacturer should be trying to mitigate. It's a risk you take when owning a device, that you can also break it, or get it infected.

Otherwise, why bother selling the phone in the first place, rather than contracting it out under a rental agreement?

So? Don't run fishy files off the internet unless you're open to the risks. Have secure walls that require either a setting change or individual permission grants before they can access secure apps.

Operating systems are prone to natural monopoly or duopoly. Furthermore there's anti consumer incentives here in that governments want surveillance data and os companies sell it.

Where competition fails to protect consumers governments must. And that includes protection from governments. I know it's ironic today as we're in a fascist regime, but that's one of the basic principles of my country. So anyways please Europe protect us worldwide consumers from American companies.

my younger sister would be in so much trouble.

your younger syster should have parental controls on, and it's worrying that you suggest it is not the case. I don't know their age but most probably they shouldn't be able to install any apps from anywhere without parent approval.

She'd have various accounts phished within a day.

guessing fron what we already know, she probably shouldn't have half of those accounts.