What's stopping you from using a feature phone?
Back to your point, there's already a "split of hardware and software" in the PC market, and we know how it works out. Security there is a joke. Windows might be getting monthly security patches, but the same can't be said of the panoply of third party drivers/firmware. Whenever microsoft tries to push for better security they get shouted down by people claiming it's some sort of conspiracy to implement DRM.
Let's not forget that all these "features" which enable corporations like Google take complete control over the project also end up driving price up, constantly. Cheap phones are a sh*t iteration of more expensive phones, instead of being simpler more basic implementations of must have features without the "quality of life" bloat on the top tier models. They should have a different tier OS rather than the same one.
I would also not make the parallel between comms devices and PCs, they're different beasts.
And a such a product is going to absolutely niche, which means no economies of scale producing or maintaining it. You try to justify that by saying it'll be maintained by "the community", but who's going to want to do unglamorous work fixing security issues, compared to developing features? Mainstream phones have dedicated security teams and freelance vulnerability researchers going after them for fame/clout. Who would want to do security research for what's essentially a glorified nokia 3310 that maybe 1000 people use?
https://danieldashnawcouplestherapy.com/blog/yellow-rock-met...