Should and can are vastly different things. It's going to be impossible for me to make the business case to my CEO that we no longer support IE10 if 25% of our business travels through there.

Some people are locked in - their offices haven't upgraded and you run a B2B site (i.e. working with a government or slow-to-adopt business), you simply don't have the luxury of throwing away business just because they're not supported.

I'm all for everyone upgrading, but reality doesn't always match expectations.