It will be a while still before you can actually party. As long as a measurable percentage of your users are still on these browsers, you're missing out on money of you don't support them.
Businesses might be missing out on customer money if they don't support obsolete browsers, but the people developing the webapp/website/whatever are definitely missing out if they don't start with 'we support all modern mainstream browsers' and then supply a separate bid for old IE support. Let the business decide whether the significant extra cost is offset by the extra income from these users.
"As long as a measurable percentage of your users are still on these browsers, you're missing out on money of you don't support them."
This seems to be a common refrain here. It's missing an important caveat: "unless supporting those browsers costs more than the customers are worth". Supporting ancient IEs means things like: avoiding features that can't be polyfilled, providing XP images for CI, debugging IE issues, etc. None of these happen for free.