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Even though it's a quite simplistic statement and we miss a lot valid scenarios to have three different environments, I would say that we fall into the norm of having 3 environments since it gives a (maybe false) sense of security and quality on our development pipeline.
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Sometimes, by removing environments, we can win quality by replacing an environment with practices like Feature Toggles, Continuous Delivery or Continuous Deployment. We might be missing good practices that drives software excellence because it's hard to challenge the norm