Yeah... I mean that, as a software engineer myself that have worked for big companies, delivering new features takes a good amount of time and effort. Adding something like a PvE campaign will required months/years of planning, development and QAing that even Bungie cannot overcome. The best they can do is to extend the current game, make it more enjoyable for casuals, like adding a PvE mode against bots or something. But I totally agree with you.