Yet that is what the Justice Department's Antitrust Division tried to do when it asked the courts to stop the recent introduction of Microsoft's new Windows 98 operating system until the legal fine points could be argued out. Antitrust law is so full of ambiguous phrases, mushy concepts and elusive definitions that it cannot really be considered law. Laws are supposed to tell you in advance what you can and cannot do, not just allow government officials to nail you when they don't like what you are doing or want you to do it their way. Antitrust cases can involve years of wrangling in the courts and many millions of dollars in legal fees, even when the plain facts of the case are not in serious dispute. That is because the terms used have no clear and consistent meaning.
Apple computers have a different operating system, for example.
If that share is very high, then the firm may be considered to be a rough equivalent of a monopoly. If a high percentage of the customers buy your product, that statistic after the fact does not prove that you controlled anything before the fact. Those same customers can change their minds tomorrow and you will be history. That is why Microsoft keeps updating its operating system and adding new features. If it really controlled its market, it could relax and let the good times roll, instead of constantly scrambling to stay ahead of its rivals and potential rivals.