Yes, having a single player would result in the most efficient design... Imagine if we only had a single mail/parcel supplier, or a single airline - they could design their network to cover all of the needs of all people in the most efficient manner. Do we really need three planes flying every hour by three different companies on the same route, or would it make more sense to fly a single large plane once per hour that can carry all of the people?
But with that comes the costs of monopoly - the ability to extort outsized prices. This is why monopolies are regulated (thing your local water/power network - they are allowed a fixed return on assets) or prevented from using their power in one industry to take over another industry.
Sweden is a horrible country from a mobile standpoint, theoretically. Our population density is 50% lower than the US (23 vs 35 people/km2). With Cell phone plans that cost 50% of the US equivalent (and keep in mind, the headline numbers on the Swedish websites are all-in, including 25% VAT), compared to the US where taxes and fees are on top of the headline prices). Yet somehow my phone here is far more reliable than in the USA. While the costs are low, the operators need to invest in their networks to keep their customers - this is what happens when you have choice.
Compare this to greyhound bus service in the USA... Yes, I know it is a different industry, but the comparison fits... Most people who take greyhound have no real options for interstate transit services. They are a captive audience. And greyhound knows this. Look at how greyhound treats their customers - as there is no real competition, greyhound treats their customers like total crap (just look on youtube for people trying to make cross-country greyhound trips). Competition drives innovation and investment. Lack of competition drives high prices and lack of investment.