I don't see how Standard Oil was a monopoly, it only ever had at most 60% of the market. And even then, oil prices fell substantially during its reign so what's the problem?

Windows for all its market share, has still coexisted with Mac OS for decades. During that time computers have become more affordable, more prevalent, and more useful and having a dominant single platform seems to have been beneficial to many industries. So again, I don't see what the big problem is.

The only monopoly that seems genuine to me is (/was) DeBeers diamonds, although the fact they have no control over the sale of second hand diamonds makes me question its authenticity.

Can't we admit that monopolies are basically a myth? Why do people keep bringing them up as if there's numerous examples of monopoly abuse or monopoly-induced misery.