I'm not sure that this works, because you can't draw the line so easily.
If there was an easily obtained unlocked device, more people would choose it than currently jailbreak. More people would make improvements and share them. The ecosystem would improve. There would be more things you can only do with an unlocked device. Which would attract more people, and polynomially more developers to the growing market. You get a feedback loop that ends with everybody buying the open devices.
The only way you get a walled garden is by successfully suppressing open alternatives, because otherwise the benefits of openness outweigh the costs and people start to move. AOL loses to the web. Proprietary Unix loses to GNU/Linux. Windows RT loses to actual Windows.
You can't get everybody to choose something where the only difference is that it prevents them from doing something they want to do. They only choose that if the choice is bound up in several other things they want and they're really choosing the other things.
But the underlying question here is not actually whether to have curation or not. Linux package managers are curated. If you create an environment in which all the good things are in the store and none of the bad things are, ordinary people have no reason to install anything from outside the store, and can then be trained to look with suspicion at anything that isn't, even if it's still technically possible.
The problem we have right now is that the assumption is false. The platform stores reject things the user actually wants, or would drive them out with high fees if there was some viable alternative installation method available. Which then drives users to uncurated installation methods, if they exist, which the platforms consequently then try to stamp out.
The better solution is to have first class competing stores. The competition solves the problem. Stores that charge excessive fees lose developers to ones that don't. Stores that reject good apps or allow bad apps lose users to ones that don't. You end up with a couple of stores that have low fees and high quality apps, giving people no incentive to leave the garden even if there is no lock on the gate.
But you have to leave the gate unlocked or the user's ability to leave doesn't cause the garden to improve to the point that the user has no desire to exercise that ability.
There are security implications of such freedoms which should be avoided of course for the regular users. The phones should be locked down as much as possible. But that doesn't have to be the only option.
This is way GrapheneOS [1] exploits Android flexibility and customization to try to improve fundamental OS security to the most modern standards and attempts to break the closesed walls of Google WHILE still shipping a fully locked down phone at the bootloader and OS levels [2].
The ideal world is Apple and Android does their normal security lockdowns for standard endusers for security's sake. Then they offer a separate hackable version where research and those crazy UI/UX hobbyists can do their thing.
In all technical senses this is entirely possible for a vendor to do. So it becomes a product question. And I'm convinced they will get ancillary benefits from the hackers doing expimentation in both security and UIs and whatever,\ from which to draw inspiration for their IRL produdcts. And possibly just as important - talent (developers, developers, ...) there are people who care about this stuff who just want to hack on their devices. These can be talented individuals and who knows some future leader within Google et al, the way the Napster guys are SV celebrities now.
I'm convinced there's enough ROI (yes commercially) for every OS platform to support this niche. They shouldn't be relegated to the ruffians and pirates on the fringes using risky circumvention techniques to achieve their goals.
[1] https://grapheneos.org/
[2] https://attestation.app/