One of the reasons Intel refused the offer was that Andy Grove had established a goal: that the company would seek a monopoly position, but would do so by legal means. It's a philosophy which didn't entirely survive his departure from the company as you may recall Intel reached a settlement with the DoJ over anticompetitive monopolistic practices (though in fairness: the company did settle and fairly quickly).
The conspiracy and means Gates proposed, along with many, many other actions of Microsoft, did in fact constitute illegal monopolization, as Judge Thomas Penfield Jackson's findings of fact established: http://www.justice.gov/atr/cases/f3800/msjudgex.htm
As George Hoar, one of the authors of the Sherman Antitrust Act stated: "... [a person] who merely by superior skill and intelligence...got the whole business because nobody could do it as well as he could was not a monopolist..(but was if) it involved something like the use of means which made it impossible for other persons to engage in fair competition."
And in particular, the statute itself states: "Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal."
So, while your hypothetical has certain merits, in the particulars of this situation, it rather markedly fails to apply, as established by both the relevant statute and legal findings.