It's part of keeping people playing and getting excited to get new stuff... every MMO has done this. Just as an example, WoW did this in the fireman's raid for ragnaros. It's just a way to keep guardians coming back for more. The galaxy in destiny is an ever changing and ever evolving thing. To keep up with changes, they'll have to make changes in the way you'll obtain items. It's no different than getting new gear that outdates your previous gear in any game with progression based group activities... i.e. final fantasy, guild wars, neverwinter, wow, Diablo, etc. Happy hunting. :)