I would say that handing out better gear, adding echo and eventually unsync are all fine because they are all optional. You can still choose the original challenge if you want it. Direct nerfs don't give you a choice.
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They could just move 210 ilvl gear to normal one, and we had same situation as with SCoB\SCoB(S), and everybody was fine with it.
GZ with two story modes. Maybe we need 3-rd story mode raid btw, why not? Let's call it impossible, that over some time would mean impossible to wipe
Also, why does content get "old"? It's not like SE needs to delete parts of the game to make space for the new stuff they're working on. If your primary concern is the challenge itself rather than the material rewards, then there is no "current content" or "old content", there's just content.