Imo phones where never used or intended to be used as computers. This is not just about monopolies; a touch interface offers a completely different UX than a keyboard and affords very different relationships with the hardware. I am not even sure if any alternative to this user experience and relationship exists (and I am not talking about geeks and hackers who are gonna make anything out of anything).
And you are not even free to switch really except between android and ios. I do not know what others here used their phones as, but for me it is a lot about essential and quasi-essential apps ranging from banking, mobile paying, 2fas, chats, and several service-related apps. A lot of these are almost essential in everyday life and not replaceable by something else. I do not want to risk to get some custom OS that offers some interoperability and discover one day that app X that I need cannot work there. I do not really use my phone as something that replaces my computer.