I have always been of the mind that the actual things of value your app does, the parts that actually make money, it does server side. That hasn't changed with single page apps and phone apps. Our back end has slimmed but the things of actual value, the things others don't offer all happen server side. For instance in our case most of the "magic" takes place by processing hundreds of gigabytes of combined users data, data we couldn't send to the front end for reasons of both volume and privacy.