15% of devices is not 15% of users. From my own experience having a web app that is 99% desktop windows users, why would I care about safari?

Maybe for your app, it doesn’t make sense. And if it’s a pure enterprise app, fair enough (assuming it’s an enterprise that was started more than 15 years ago and only targets regulated or very specific markets). But a good way to guarantee that your app will never go beyond Windows desktop users is to ignore the most dominant mobile platform by users who actually pay for software.

Well, if your app doesn't run on Safari, you're not going to have any users on Safari.