There seems to be a debate whether or not you should plan for the best, or plan for the worst. 

Planning for the best seems like you’re being shortsighted and overly optimistic. 

Planning for the worst seems like you’re setting up low expectations. 

Really then, we should be planning for both outcomes, so no matter the extreme, we’ll have either a robust plan of action that can scale up or down, or multiple plans that you can switch between depending on how things are going.