From merriam webster:

    1a : no longer in use or no longer useful
         // an obsolete word
     b : of a kind or style no longer current : old-fashioned                 
         // an obsolete technology

A good example is a satellite, lots of older (obsolete model) satellites are still in orbit and performing well, and replacing them is not worth the cost. But if a new satellite is needed the newest model will be send up, not the obsolete model.