In just under 12 days, Apple is going to introduce a new look and feel for Mac OS X. This is going to be much more disruptive than it was for iOS 7.
On iOS, an app takes up the entire screen and becomes an extension of the device. It’s also something you’re “in and out” of on a regular basis. It’s not the end of the world if one or more of your apps still looks like iOS 6 because it’s not competing with anything else and the pain is short-lived. Bank of America’s app is still on iOS 6, but I use it infrequently and usually just to deposit checks.
But on the Mac, an app usually shares the display with other apps. And they’re something that you use continuously. If a customer’s Email, Web, Calendar, Contacts and other built-in apps are using a fresh, new look, they’re going to feel put out when they switch to your app and get a legacy appearance.
You will stick out like a sore thumb.