It's hard to find an example, because no company is stupid enough to push out all the competitors. Instead they push out most of the competitors, while keeping one or two around (so they don't get into trouble with monopoly laws), and building big barriers to entry for anyone else.
Then they can increase prices a bit, although the main profit source is in reducing unit costs now you have a big business and making all your profit through volume.
It's certainly non-ideal for customers, but at the same time I think customers usually get a better service for a lower price than in a world with hundreds of competing companies (where overheads work out much larger)
What would be the ideal situation? Like you said, having too many competing companies doesn't lead to great services at low cost because of the extra overhead and lack of volume, and having a monopoly gives one company too much power, then isn't duopoly a nice equilibrium?
Then they can increase prices a bit, although the main profit source is in reducing unit costs now you have a big business and making all your profit through volume.
It's certainly non-ideal for customers, but at the same time I think customers usually get a better service for a lower price than in a world with hundreds of competing companies (where overheads work out much larger)