I am just going to wager that you didn't do anything to accomplish much with your game time when you played if you are sincerely asking this question, and to be honest it's not an easy thing to try to explain to someone how having all your game effort made into wasted effort is annoying, if they have no idea what it means to accomplish goals.
But to be blunt, I did not spend time farming "this years" best in slot items, if I went after what had been the best items several years running, and as I farmed for them, they remained the best items, if at any point I knew they were going to be turned into temporary trash, I would not have bothered myself. It only serves me well, if the effort justifies the reward. In this case, having the item become invalid, eliminates the justification for the effort.
Turbine then turned around and made what had been very long standing items into "this years" items, which means, there really is not point to bother farming for them anymore.
Now, beyond just the sheer insult to the players themselves who had farmed the gear, you may have missed a bigger point, the quest did not go away, the loot from it just got turned to trash. So the quest remains there, the loot remains there, and it is all now wasted memory space. There is no need for me to get it for an alt, there is no need for a new player to quest after, it's trash, junk, there is no reason for someone to buy the pack, or do the quests at this point in the game, and that is about as bad a design decision as you could make for any game, to make your older content not worth running. That is what putting in items with longevity ensures, that your content, from the first dungeon to the last, gets run, that players want to run them, that players want the loot from them. That new players are moved to farm them for their own gear sets, and the older players show off what you can get and what you can do with it. After all they have had the time to optimize their gear sets.
In today's game, I an not going to spend a month farming an item that I will be throwing away in a week, that is a waste of my time. The truth is, only by making items with longevity do they become farm worthy, only items with a long life are items worth having, anything else is not worth more then a passing glance.
It is one thing to move up on the list: IE: You have the best items for level 25, and then peruse the best items for level 28. That is an acceptable pursuit.
What is not acceptable is to have level 10 items getting invalidated. I did not farm all this gear over the course of the last few lives and years to have to farm gear again. I did that work as prep to be able to just log in and play and not worry about gear anymore. It was never intended to be "this years" gear for me. If I felt the urge to keep farming gear, I could always go after a few remaining items that I don't have or make and outfit an alt, at no point did I ever want to have to re-farm gear for the same toon.
So maybe you don't understand that mindset, well, you're not alone, it seems neither does Turbine, and the numbers show they are losing players because of it, just like you would if you had your hand in designing this game.