I get it that people want more freedom from their iPhones but the thing about consumer devices is that they are an expression of a certain philosophy of how computers should work. Being a walled garden is one such approach. If you don’t agree with how a device operates on principle, you should not buy it—there’s Android or derivatives. You’re also likely to be a power user who’s in an incredible small minority because iPhone sales keep getting better every year and the walled garden approach has market (as in free market) validation.
Now, if your objective is to regulate monopolies, I think that the policing should happen in the supply chain and production side instead of the consumer software side. You don’t have more options than iPhone and Android because big players like Apple and Samsung have captured manufacturing facilities with long-term exclusivity contracts, making innovation in the space prohibitively expensive. But the law shouldn’t dictate what sort of computer innovators are allowed to build.