Easy, because it is a common archetype of player. Previously this was handled with raiding, with 4 encounters that were the hardest in the game. Now raiding is more of a mid-core piece of content, with the hardcore crowd getting one encounter. It is not a waste of resources to cater to hardcore players, as much as it is not a waste of resources to include a massively difficult setting in any game. The fight in question is likely using no extra assets (and definately doesn't add any story), and is simply changing an existing encounter, don't be selfish.
I will ask though, isn't it more of a waste of resources to undo the content's intent after a 3-6month lifespan, than to keep the challenge there for the players that want it throughout the game's lifespan?
It is nothing like that actually, because the idea of outdated content is manufactured, and only really applies to progression content, which the super savage fight is not. Is PotD outdated? no, because people still run it for levelling, are ARR hunts outdated? no because allied seals mean something. So why should super savage automatically be outdated?
Titles/mounts/minions/glamour last forever, and there are no numbers attached to ever say they are outdated. If SCOB hadnt been messed up by level sync, I would still be in awe when I see someone with a SCOB savage title (as it stands, not so much now)