As long as they didn't become a monopoly in any specific creative field, it would be fine too.

A literal monopoly is not necessary for a court to determine that a firm can exert significant and durable market power over its competitors and customers.

If a firm is able to achieve and maintain a monopoly solely through lawful and reasonable means, such as producing and offering a superior product at a lower price, it is free to reap the benefits of that fairly-attained monopoly.

That constitutes monopoly and anti-competitive business practices.

But this isn't the real issue. The real issue is that the US hasn't vigorously enforced its own antitrust law in over 20 years, because a policy of "monopolies are good" was adopted in the 80s and has only gotten stronger over the years. (The Microsoft antitrust case of the 1990s was an exception.)

I don’t see how this behavior would be particularly different from John Rockefeller buying up all the means of oil production.

There's also the question of how long would this work. This would stimulate windows DAW market; every software shop would want to sell the next mainstream windows DAW.