It all depends on WHY someone wants a waterproof bag.
If it's to camp without a tent, then you do need a tent substitute.
If it's because you are afraid your sleeping bag will get wet inside your tent. Then a bivouac or poncho would be of no help. But really, if you keep your bag inside a waterproof sac, only open it inside the tent and don't wear wet clothes in it and why should it get wet? If it does you need a better tent not a waterproof sleeping bag.
The Sock
_________________________
The world is in haste and nears its end – Wulfstan II Archbishop of York 1014.