Master Plo could survive in a vacuum without a suit and kotor had a power to breath poison with no problem. That doesnt mean most characters have that ability. To most players its a quick 1 shot kill akin to playing D&D with jason foxx in the comics where his sister spends a long time making a character and she enters a cave and he says rocks fall on you and you die. Not the sort of GM I would ever play with twice.
You flip a destiny point and just grab your reserve breather, besides destroying an important piece of gear requires a triumph to begin with.
If you want a cheap one shot kill you go in with a vibro axe and 5 ranks of lethal blows. ;-)
Losing your breather is inconvenient, but no way in hell a player character will just die because he might or might not take a single breath of toxic air. It star wars for pete's sake, you get thrown in lava and just need a few cybernetic upgrades. Lungs can get replaced, bacta is a heal all, cloning tech is avaible to grow new limbs and organs, etc
The assumption that a broken breather is automagical lethal is questionable.
To most players a broken breather is the loss of two maneuvers next turn to pull the reserve breather out and not even a destiny point needed as most players will indeed think about bringing spares of a piece of gear required for breathing.