> if this _were_ a mainstream concern the market _would_ self-correct.
The underlying premise here is that the alternative is available for consumers to choose, i.e. that you can buy something which is otherwise equivalent to an iPhone but supports third party app stores or installing a third party OS. But that isn't the case.
What you get instead is e.g. Fairphone, which has the specs of a $200 phone but costs $800 and if you actually have root your bank app might break etc. And still many people buy it. So all you can conclude from this is that the price the mass market places on freedom is less than $600 plus some non-trivial usability issues, not that they value it at zero and don't care about it at all.
On top of this, it's a threshold issue. If the median phone was rooted, people would develop apps that need root. When the percentage is some low single digit if not a fraction of a percent, they don't, and then taking the trade offs of a phone that can be rooted isn't buying you what it should because you need a critical mass in order to achieve the expected benefits, but you need the benefits in order to achieve the critical mass. This is the sort of situation where a mandate can get you over the hump.
> There are costs to any regulation, and lots of possible unintended consequences.
A good way to handle this is through anti-trust, because then you can do things like exempt any company with less than e.g. 5% market share. That means not Apple or Google or Samsung, but if there is any major problem with the rule then the market can work around it by having 20+ independent companies each provide whatever it is that people actually want. Meanwhile that level of competition might very well solve the original problem on its own, because now a couple of them start selling unlocked devices without any countervailing trade offs and that's enough to make the others do it.