Important or not, if your big corporate customers are willing to pay you big bucks to support it, how can you refuse? But yes, it does feel very counterproductive, and even more so over time, when you see that more and more of the popular libs drop support for ES5.
Wheelchair users are not even close to 1% of the population. Would you tell a person in a wheelchair to just use their legs when at your establishment?
I feel sorry for the people who continue working on IE because they think it's just as important as Edge/Safari/Firefox. :(