People discard relationships for all kinds of reasons, but as you get older, you recognize what a foolish act that is most of the time. Good relationships are valuable and to discard all that history, in most cases, is a true waste of potential and support. That said, there will be times when discarding a relationship altogether is necessary, but those times are few and far between.
Relationships will change as will the amount of time you spend with the people in your life. Change is a constant of all of life's dimensions, and if we embrace that change with an open mind and willingness to revise so that we maintain what's good, we will do well.
So, for the most part, rather than discard a relationship, simply change the quantity of time or the things you do together--revise the relationship. This maintains the history you've shared, and opens up the potential to keep that relationship strong and meaningful over time.