They lack the numbers, the fleet size, and the worlds The Imperium has. They also lack any Exterminatus Fleets as far as my knowledge leads me to believe. They might survive, but they wouldn't ever be able to return to their former strength.
That, my friend, is a Bio Titan - The largest creature in the 40k Universe. It not only recycles corpses into biomass, it is also capable of decimating entire armies on its own! It's also one of my personal favorites.
I've only seen ONE picture that depicts the kroot actually doing their job and fighting the close combat for the tau. And that's against the guard! The most useless, chance-less force to field against the tau. Why are there so many idiot commanders in the Tau empire!?