edit: plus from what i've read i'm not even sure they didn't have fire, in the first place.

I never said they didn't have fire; please read what I wrote.

You can't just go around dispelling advantages of big brains without providing any alternative explanation why big brains evolved.

Of course I can.

And besides, it's a very simple story: if big brains improve individual reproductive fitness by enabling innovations, then there need to be innovations. Evolution can't act on genotype which is never expressed in phenotype. In Tasmania, not only are there no innovations, there's actual loss of existing innovations. If you try to argue that the population base was too small, then that's even more damning: what kind of innovation-supporting gene can be selected for by Evolution when thousands of aborigines over many generations all fail to produce anything? Even if one aborigine did, how does that make up for thousands of his relatives/ancestors burning huge amounts of calories and protein on the innovation-capacity only one of them benefited from?

Combined with the observations about innovating being a public good (and group selection being rare to non-existent), this is pretty convincing evidence that big brains were not selected for their innovation, but for something else - the Machiavellian brain hypothesis seems like a good one.