In Denmark you see this in action in the telecommunications industry. A new company is started, is more adaptive to consumer needs, is more efficient, takes away business from the big players, and eventually sells to one of the big players for a lot of money because the big players are losing a lot of revenue. New entrepreneurs notice an opportunity and then does the same.
What would happen if all the big players said to every newcomer "screw you, we're not letting you connect to our network at all, good luck selling your service if your customers can't actually talk to anyone with it"? The newcomer wouldn't last long. This happened in the US to a certain extent, by the way, and is why there are telecom monopolies there.
Fortunately, it appears the telecom industry in Denmark is quite heavily regulated.
They would lay down their own lines in a localized area and expand from there.
The US has telecom monopolies because the US government literally gave AT&T a monopoly and only removed it recently (relatively recently, anyway). Also, various regulations regarding the last kilometre, starting a telecom company, etc.
At the turn of the century, the US had thousands of telephone companies, and AT&T only had 51% market share despite having started with practically 100%. There was plenty of competition, and remember that this was back when they had just been invented. Then the US government took over all of the privately run telephone lines and gave them to AT&T.
There was also competition among electric companies, despite electricity generation being an entirely new industry.
There is no such thing as a natural monopoly.
