Well for me SAI_Peregrinus worded it better: "A key realization is that people don't buy computers (and phones) to run operating systems, or to run software. They buy computers to accomplish tasks. The software isn't what matters, completing the task in the manner they expect is."
Speaking to my non-geek friends, they simply don't care that their phone can't run every piece of software out there, they only care if they reach for their device and can't complete a task they wanted to do in the manner they expected to complete it, and I could prob count on one hand the times I've had people come up to me over the years with a question about a task that couldn't completed within the eco-system of their phone.
You said "Nothing you said seems incompatible with an open device. Apple could open/unlock your device tomorrow and it wouldn't change a single thing you said." And that is true, Nothing I said and have seen in the wild is preventing Apple/Google/Samsung releasing a full open phone, apart from that the vast majority of their customer base (at the moment) simply does not care if its open or not so manufacturers have no reason to be open.
Even if I ask them about cost of software and suggest to them that the cost of their apps could be lower if they downloaded them directly from the dev or even from a 3rd party store they don't seem too bothered even then, I mean look at the game console market of another example of consumer apathy about open eco-systems. Though tbf, due to the Steam Deck I have seen a bump in "Oh, I can run pretty much anything on it? even those old childhood games I loved via emulation?" and "Oh I can purchase my games cheaper via key resellers?" once they get a hold of it, so maybe the world isn't as lost as it seems to be.