Obsolescence can mean a lot of things. Here is some information for you to digest. From a financial point of view, an obsolete part is one that does not yield a return on investment; yet it makes the customer happy that you have one.
Obsolete, perhaps, but if a part sits on the shelf (the part being similar to cash on the shelf or a used car on the lot) there comes a point in time when the part (used car) simply must go. There are a lot of estimates, but the one I find that makes the most sense is that when a part has gone 12 months with no sales, the total holding cost (total cost of running the parts department to support the inventory IN the parts department) is about the same after 12 months.