oh wow, you must not have a lot of apps. I don’t think Whatsapp even run on iOS 6 now and that is a communications app that can run on even feature phone. And if you don’t use any other apps except stock, you probably don’t need to upgrade?
And yes, XS is super fluid… you should try it out in a store. It’s night and day. Edit to add that I came from 6 Plus (on iOS 12) to XS Max.