"If you stick mainly to downloading apps from the App Store, your computer will never be infected." That's a bold statement, it happened before it may happen again.
Many big corporations use Windows as their main operating system, most of the internal websites only support IE. I worked for a company which had over 200000 employees. I don't know how many of those employees worked in the R&D department but in those departments the security policies were very strict, you had to request every piece of software and IT will install it for you. Development sometimes was done on local virtual machines running Ubuntu, they were also secured and you didn't have sudo access except for mounting and installing packages.
All internal websites were designed so that they worked with IE. If I wanted to go on holiday I had to access the HR application which worked only in IE.
My guess is that there are a lot of legacy applications in the corporate world and migrating them from IE compatibility to something else is hard and costly.
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