It's all about for who's benefit the product is built. I like the phrasing of having a phone "on my side".
In a recent thread here on obsolete hardware I was commenting how up to the fairly recent past products were built with the idea of competing on features the user wanted (or at least presumably wanted).
But no longer - in the last ten years that has shifted in too many industries to be all about features that the company wants (spyware, tracking, DRM, cloud lock-in, and on and on). The phone duopoly is a poster child of this evil trend. Neither platform puts the user first and into the drivers seat.
So no, I don't want an open source phone nor a Linux phone, specifically. (Not saying I'd mind if one or both are true, that'd be cool, but not a deciding factor.) I want a phone where I the customer are in full control of what it does. I must be able to override anything the OS wants to do and to override anything any app wants to do.