This is a little bit disingenuous - it wasn't any old WoW mount, it was the [Celestial Steed] - the very first microtransaction mount offered in WoW. It was a skin (admittedly a very pretty one) for a new mount mostly using already-existing resources. The cost of development for the Celestial Steed was practically nil because all of the tech had already been built and all but the skin itself had been created already - the animations, the sound effects, and so on. The marketing budget was also very small - The Celestial Steed sold for $25 each and it launched in 2010 at the peak of WoW's popularity of 12 million subscribers. It sold like hotcakes and 100% of that revenue went to Blizzard. The Celestial Steed sold so well that it convinced Blizzard to continue selling microtransactions and develop more.
Compare that to Starcraft 2, which sold for $60 and had a development team of over 300 people that worked on the game for over two years. If you assume the normal "napkin math" cost of an employee (roughly $10,000 per month in total costs, 24 months of development time), that's a rough budget of about $72 million. Starcraft 2 needed an enormous marketing campaign as well, easily at least that much again. Blizzard also had to share part of the proceeds with the retailers, so a significant percentage of physical sales paid out ~20% to the retailer. This means that Starcraft 2 needed to earn around $150 million just to break even.
Obviously, not all microtransactions sell that well. It was one of those moonshot black swan events that no one on the development side ever expected to happen, so unexpected that it altered the course of Blizzard's financial future and convinced them to keep doing it. The Celestial Steed was released at the peak of WoW's popularity and sold a thing that many, many people wanted at a time when very few were selling microtransactions in that space. Sometimes people just want more [Skyline Stuff], and they're willing to spend on things they like. As long as players keep spending this kind of money on microtransactions, we're going to keep making them.
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