If they end up creating an Exotic that just throws balance out of control I'd have no issues with them sunsetting it if and only if they can't properly balance it in whatever piece of content it's out of control in.
Aside from that there's zero reason for sunsetting any kind of Exotic.
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If that happens I would call for a rework or balancing, there is no way that they can't fix an exotic, and if it's really that bad, just nerf it so much that it can't be used. Sunsetting implies large scale retirement, which shouldn't be done with something in which we have such a small loot pool, like exotics.
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They could have reworked it though, the big problem with it was how it broke the ammo economy. If they removed that and just left it as a fast firing full auto fusion then it would be annoying, but not broken.