Competition is how you get to the best product. Lack of competition leads to malaise of product improvements as the market dominators are owning the space and happily exert their power over people.
> It's no different than going to a restaurant and them serving only Pepsi products.
There are two viable players for the average Joe in the phone market. There are I would guess 200-300 restaurants in my not so big town.
The number of choices matters a lot. If there were only two real option for restaurants around me, I would hope the management does not decide to be evil and lower food quality, jack up prices, or collude to only offer specific food while the other restaurant does not offer.
Also, in the restaurant example, we always have the option to buy our own food and cook at home. So to match the phone market situation, imagine cooking at home is illegal, and the only food you can eat is from two restaurants.