Flexbox is great. Now we just need to wait a couple of years for 99% of the active browsers to be upgraded. I know there are shims but I cannot imagine they don't come with caveats. Web development has come a long way but it's a slow process. Remember when we had to support IE6. (shudder) Now most people are ditching IE8 but 9 is still common.
Sometimes IE7 statistics based on user-agent strings is unreliable.
We have a phantom measurement of 3% IE7 usage when the actual usage is 0% (our web app prevents IE7 users using JavaScript because it is too broken). The measured IE7 users are actually using IE8 (as a control embedded within a Windows exe, actual usage measured using @cc_on @_jscript_version).
