It doesn't matter whether the monopoly position was achieved "fairly" or not. Either way it is detrimental to the overall health of the economy and to consumer choice, and that alone is good enough reason to bust them. Those laws were passed for a reason - people saw what the end result looked like, and they didn't like it.

If you don't want someone to come and bust a monopoly you've spent years and billions building, then don't use your time and money to build a monopoly.