I would try communicating to my manager that maintaining IE 7 support is costly for you. When you work on some IE-7-specific issue, log your time, and show them an actual rundown. Maybe that will tilt the cost-benefit analysis far enough to push them into upgrading their default browser.
(By the way, I'm sympathetic with you. When I deployed a big feature in an intranet web application in 2012, it broke for dozens of users, and it took me days to track down the misplaced comma that broke the neck of the IE 7 JS parser.)
More than 69% of our regular users are on IE7, we just can't afford to drop that. (They do in fact get a pop up telling them to upgrade, but that had no noticeable affect on traffic).
My comment with browsers, is more about knowing your audience. Most of the evergreen browsers do support flexbox but...
Well, I maintain a website for a company.
97.3% of visitors, are return visitors.
69.77% of the visitors are using a version of Internet Explorer.
69.03% of the visitors are using Internet Explorer 7.0.
I have no hope of getting to use flexbox to simplify managing the website, which makes me very sad.