Imagine that your football team wins the championship the last years and it has completely crashed all competition. Would you be happy with the way your team played with no competition at all? It would surely play crappy, because it wouldn't need to try any harder. Now imagine that your opponents got stronger by bringing talented players, accomplished coaches and investing in their infrastructure (academies, training grounds, etc). Your team would be hard pressed and improved to keep ahead of its opponents.
The same thing happened with windows.
Having more than 90% market share for years, they got slower, more bloated and less innovative.
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