Not unreasonable questions! The reason we want to fix the flurry bug has two major factors: The first is that the flurry bug is unreliable. It doesn’t happen all the time, nor does it happen for all players. It’s not a “fair” bug. The second, is that the flurry bug has a massive performance cost. Projectiles are one of the more costly things for both server and client performance as they need to track and check collision along their path as they travel, then calculating damage when they do interact with a hitbox. For server performance, it also has to share projectile information so other clients can render the projectiles. There’s certainly performance tricks at play to minimize this, but with the shear quantity of projectiles, it still adds up to a lot.
As for “Why not a corresponding buff with the fix?”. This is because discussions need to happen regarding the skill as for why it’s behind in popularity, what the best way to go about buffing it is, getting those buffs in, testing them to see if they achieve the goal, then finally releasing them. In essence: “time”. We have to make decisions constantly regarding where our resources are spent, and when. Spending time discussing and looking into flurry and working on buffing it now means we don’t have those resources now for fixing other bugs, getting Item Factions ready, etc.
We certainly understand the feeling of a build getting nerfed after spending time building it up, and getting items for it, even if it’s because of a bug, and we do want to avoid changes which heavily impact builds mid-cycle. We also have to make a call as to when we have to. In the case of flurry, not being available to all players, not being reliable in how much effect it has, as well as greatly impacting performance on both clients and servers it falls over the line on a bug we need to get a fix out for.