In many ways this is a "classic Microsoft" move — take something that works perfectly fine and replace it with something new but completely inferior and often less featureful. They have been doing this in the consumer space for years and inevitably users get upset but move on.

What I doubt they have considered is the number of businesses dragging-and-dropping mails as EML files and working with offline PST folders and relying on various other invisible features daily that will be completely broken by the transition, not to mention that many businesses likely won't be as tolerant of that breakage as the average consumer will.