The market share varies dramatically by niche. If you're running sites aimed at developers, you'll get very different results to a site aimed at teens on mobile, or one aimed at large blue-chip corporates.
Supporting IE < 11 shouldn't be a technical decision, I'd say. It is simply not technically difficult to do (outside of a couple of very niche web technologies). It is a business decision. So, you need to ask the CXOs if they're happy to leave those customers to your competitors to save you however many hours of dev and testing. There's a point where the answer will be 'yes'. Mostly we've passed that for IE6 now, but not always. I'm aware of one company targeting government clients, particularly in the developing world, for whom IE6 support is essential.
Supporting IE < 11 shouldn't be a technical decision, I'd say. It is simply not technically difficult to do (outside of a couple of very niche web technologies). It is a business decision. So, you need to ask the CXOs if they're happy to leave those customers to your competitors to save you however many hours of dev and testing. There's a point where the answer will be 'yes'. Mostly we've passed that for IE6 now, but not always. I'm aware of one company targeting government clients, particularly in the developing world, for whom IE6 support is essential.