The law of optimums states that some industries are naturally more efficient with only one supplier. Specialized computer software for health care, for example, will only have a market with a few hospitals worldwide, and it is more efficient to pool all resources with one company. Such a company would exhibit increasing returns to scale.
Some industries will suffer from scale however. A hair salon gains nothing from merging with other hair salons except increased management overhead and reduced profits. This industry remains more efficient with many independent suppliers.
The idea that you can "improve" competition is bogus. In a free market with no coercion, competition is the natural state of any business, no matter how big or how alone it is in its sector.
Here's a short and enlightening essay on the subject, published during the Microsoft trial:
$this->bbcode_second_pass_quote('', 'T')he Microsoft trials remind us that the fear of industrial concentration is the last refuge of socialist theory. The claim is that capitalism ultimately fails because all (or most, or at least some) industries naturally congeal into monopolies in a free market. It follows that government must regulate industries to bring about "competition." It also follows that since some people in these giant private industries become unpalatably wealthy, it is fair to confiscate their personal wealth and give it to people who are less wealthy.
The assertion that free markets lead to monopoly is wildly incorrect. If the market is allowed to work freely over time, an apparent monopolist soon discovers that it indeed has competition. A company operating in a market economy looks like a monopoly only under myopically static analysis. A broader definition of any industry will show that there is plenty of competition, just as a narrow enough definition will show that any brand name product has some monopoly characteristics, such as a popular brand of ice cream.
The airline industry is an example. There are now two manufacturers of large passenger jets: Boeing and Airbus. Punditry has expressed inevitable fears over monopoly profits and passenger safety. However, Boeing's actions in the last three years--most notably, attempts to cut costs by modernizing the entire production process --suggest that Boeing believes it has competition.
Boeing is right, and the competition is not just from Airbus. Suppose Airbus closes its doors, and only Boeing remains. Suppose also that Boeing faces no government regulation. Can Boeing raise prices at will? If it does, in the short term, people who have to travel will find alternatives to air flight in increasing numbers. Airlines would use smaller planes as much as possible. In the long term, companies such as Beechcraft and Cessna, seeing higher than normal profits available to an interloper, might build larger jets.
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"Fear of Monopoly" -
http://www.mises.org/story/621