Why don't our poisons last forever, like death knight weapon runeforging?
New rogues have no idea how easy they have it. Back in my day, rogues had to hike uphill in the snow to a special poison vendor hidden deep in the bowels of the capital cities. Once there, we had to look at our shopping list of obscure reagents to figure out what to buy and in what quantities. Rogues had to combine these ingredients in vials, creating one poison dose that faded every time we zoned. Vanish also consumed a vendor reagent, and we had to farm up
The fact that poisons no longer consume bag space in Mists of Pandaria was considered to be a boon. Poisons are easier now than they've ever been before.