You shouldn't trust them to be nice, but you should trust them to be greedy. A policy that sets the seats so that only a very small minority has to buy two makes sense from a greedy standpoint. It makes sense from a customer standpoint as well. Over the long run, more seat room means more fuel consumption per person and lower loads, and that means higher prices. Passengers complain about comfort, but the vast majority of people (especially on shorter flights) buy the cheapest flight no matter what. So most people end up wanting the smallest seat that's perfectly fine for them. (UA's been trying to change that mindset by getting average people to be willing to pay for more leg room.) Just like how people complain about paying for meals but most won't pay an extra $5 per ticket for a flight that comes with a meal.

A policy that sets the seats so that the majority of people couldn't fit would be a sure money-loser, just like a policy that makes all the seats large enough for the type of passenger who comes on a flight once or twice per year.