> we don’t want to have to force smartphone choices but the number of users without one these devices is so vanishingly small

You are missing the point. The issue is that once the "vanishingly small" number of alternatives disappears, users will be completely trapped, and Google and Apple will then free to abuse that position of power (they already do). Worse, since power is centralized, it is very easy for government interference to take place, and we already see that with things such as identity and age verification requirements. It is the possibility of competition that matters more than actual competition.