IE6 was ready to die the moment it was born. This story boils down to some influential developers saying "Fuck this shit" and doing something about it, taking most of the industry with them.
Given that IE6 was one of the worst pieces of software in all of human history, there was never likely to be a significant downside.
That is untrue. When IE6 first arrived it was on par with Mac IE 5.2 in terms of web standards support. It was a great browser for its time.
The problem with IE6 is Microsoft declaring victory in the Browser war with Netscape, and IE6 development stopped. For years until Firefox and Chrome threatened Microsoft's browser dominance.
Given that IE6 was one of the worst pieces of software in all of human history, there was never likely to be a significant downside.