I considered that possibility as well, but rejected it because when I scoured the web for reviews, several people reported that their soda would remain fully fizzed for a least a week. Assuming they're not lying, that would tend to exculpate the sodastream machine itself, leaving just the...
Sodastream sells a device that will charge a 1 liter bottle of water with CO2 gas so you can drink fizzy beverages. I always use 40F cold water from the fridge to fill the bottle, and seal it tightly, but I find that after only 1-2 days 90% of the fizz is gone, a much shorter life than fizz...