The only reason why poison damage did not stack back in the original Diablo 2 was due to limitations in processing power from the era of Windows XP.
FULL STOP: You do not have an excuse to not fix this!
Fix the poison damage!
If I am using Poison Dagger, every time I stab an enemy, all it does is reset the timer on the poison, and when any damage is done to an enemy while it is poisoned, the enemy fails to take poison damage for that frame.
This is broken enough to ask for a refund on the game purchase, fix it.
Project Diablo 2 proved that poison Dagger Necromancer can be a viable end game build, they fixed the programming and they are not even ran by a big name company with a big budget.
FULL STOP: You do not have an excuse to not fix this!
If you want to claim that fixing this would require money you just don’t have, please tell me how much money it would cost to fix this and I will personally start a crowdfunding campaign to see to it that you can gain the funds to fix this, I am that serious and passionate about this.
FULL STOP: You do not have an excuse to not fix this!