Complexity is always the thing that needs to be managed, as it is ultimately what kills your app. Over time, as apps get more complex, it's harder and harder to add new features while maintaining quality. In a greenfield project you can implement this feature in a day, but as the app becomes more complex it takes longer and longer. Eventually it takes a year to add that simple feature. At that point, your app is basically dead, in terms of new development, and is forever in sustaining mode, barring a massive rewrite that dramatically reduces the complexity by killing unnecessary features.

So I wish developers looked at apps with a complexity budget, which is basically Djikstra's line of code budget. You have a certain amount of complexity you can handle. Do you want to spend that complexity on adding these features or these other features? But there is a limit and a budget you are working with. Many times I have wished that product managers and engineering managers would adopt this view.