“If there’s truly a market” monopolies, monopsonies, and anti competitive behaviour would never be a problem.
What is either company doing unlawfully that’s preventing a disruptive smartphone competitor from arising? Being a duopoly is not in itself unlawful unless either is taking active steps to prevent others from competing in the market.
A truly open phone is just a pocketable Linux laptop with an LTE modem. These already exist, companies like GPD make them.
I’m not sure something with a form factor of half the size of a sheet of office paper would suit the market for a typical smartphone.