Doesnt matter, this exploit was actually being deployed through a word document, which bypassed the need to actually use IE to be vulnerable.

Enough people, especially in businesses, that removing it would be catastrophic for Microsoft.

I know several companies who developed internal applications using Silverlight. They still have to use Internet Explorer.

The least number of them use it voluntarily. It's usually in bigger corporations where it's set as default browser and even if you have an option of installing a different browser, there's no way of getting into the intranet with that.

Anyone who still hasn't migrated off Windows 7 or (for whatever reason) 8.1.

answer is right there in the article: Operating System