Of course there is the fact that humans arent QUITE as numerous as they used to be. So the oxygen should last for a while. However, in one of my games, I pretty much lost all of South america to the mass... and what does south america have? Lots and lots of trees.
After loading one of my savegames in tactical I discovered an annoying bug. The camera kept rotating in a clockwise motion. Nothing I do can stop the rotation. As you can imagine, this ruins the rest of my game.
The biomass starts spreading itself all over the planet, converting plants, animals, anything into it's own biomass. By the time you get the technology to fight this monster, at least 30% of the earth's lands should be covered. And then you start thinking... The plants that were under the mass itself as well as the animals should all be dead. How on earth are we going to survive when we no longer produce oxygen and have lost most of our food supplies?