Well, depending on the incentives and restrictions involved, an ecosystem of 3rd-party builds is a potentially viable escape hatch for the user from the manufacturer's grip.

Of course the sticking point is the degree to which the hardware requires proprietary and opaque binary blobs in order to enable important user-facing features. But then, that isn't anything really new, as open source PC operating systems have been dealing with this issue since forever, with the caveat that PC hardware is mostly modular, so having or swapping in well-supported components is an option, whereas smartphones are an integrated slab of metal, plastic, and glass, with "no user serviceable parts inside" as the status quo.

But even that caveat has precedents, in non-PC devices such as consumer networking gear that only became well supported through aggressive GPL license enforcement actions that freed some of the necessary code.