This certainly seems to indicate that great product diversity is not necessary for strong financial performance in the short to mid term. I think the question here is more one of longetivity - how long will this "one trick" remain relevant, and retain a large enough market to keep increasing the profits?

All those oil companies would run into a lot of trouble if, say, the oil ran out. On the other hand, Microsoft could keep generating substantial revenue even if people stopped buying the XBox for example. Their other (major) products such as Windows and Office provide enough diversity to keep them relevant, despite the loss of one or two major markets.