The most infuriating part about the Sym3 rework is simply that Sym2 went a full year with no adjustments at all. All the received during this full year was bug fixes, mostly unwilling created in the previous patch (eg, Moira orb damaging her turrets, or her ult breaking for no reason immediately upon placement).
Sym2 was far from perfect, but she had room for adjustments. Every Symmetra players had their own list of desired balances for her (here is mine). But Blizzard just recreated her kit from scratch based upon the same initial concepts instead of making any adjustments to her previous kit.
IMO, this was mostly because her in-game stats showed a strong hero that was queen of winrate since launch, and had the most OP ultimate in the whole game (Shield Generator), yet the community always perceived her as a weak pick. So the devs knew they couldn’t buff her (because stats) neither nerf her (because community), so their only option was a full rework.