What plastic are you thinking of and at what thickness, that is compatible with liquid oxygen, retains flexibility at LOX (or worse, LH) temperatures, and withstands the pressure, all without adding a massive mass penalty? How is the plastic supposed to deform around every little structure in the habitat (aka, not face multiple atmospheres of asymmetric pressure)? What sort of hardware are you thinking of where every last element is just fine with being frozen down to LOX (or worse, LH) temperatures? How many man hours are you thinking of to "rip out" the giant bag through the tiny docking port (after having to detach it where it's carefully bound around each element? Unless you were thinking of having it fully loose inside there, which is even more problematic. Where's it supposed to go on the ISS? If you're doing the (larger) hydrogen tank, how 100% sure are you that you're not making an explosive fuel-air mixture, given that hydrogen burns at just a couple percentage concentration? How positive are you that you've fully vented every last nook and cranny? And on and on and on.
Wet workshops were worked on during the Apollo era. They were ditched for dry workshops because it's easier, cheaper, and more functional.