What other companies? Do you know the definition of a monopoly? If one corperation controls an industry, upstart companies can't compete.
What you're thinking of is a environment like NOW. Yes, in a regulated environment, no single company can have the whole cake because it's against the law, BUT in a FREE MARKET, the monopoly eats the WHOLE cake. There is no such thing as a start up company because there is no market share for that company. You can't start off a company with 0% market share and expect to expand.
Bear with me as a provide a physics analogy.
Consider:
light travels at the speed of light.
light cannot decelerate from the speed of light.
nothing can accelerate to 100% the speed of light.
analogy, as per your statement above:
Monopoly holds 100% market share
Monopoly has no competition and thus cannot "decelerate" from 100% market share
nothing can "accelerate" to 100% market share.
Thus, monopolies only exist if they were preordained (regulated), much like light, to exist as such.
In fact, it is impossible for a company to become a monopoly in a free market, much like it is impossible for an object to accelerate to the speed of light.
A silly analogy for sure, but it is apropos.
Follow the logical process: if one company achieves massive wealth from a particular upstart industry with very little competition, many many more companies will try to improve on their company business model and enter the same new upstart industry because it is lucrative. That's logical. It's also exactly what happens. For example, consider the boom in social networking websites, search based websites, reality TV, etc.. One successful company in a new industry naturally leads to more competition in that industry. Thus, no monopolies.