That's not true. That's like saying "If we had refrigerators, we could keep food good forever". As you know, food in the fridge can still go bad, because no technology is perfect. I think that terraforming technology will be an extension of techniques we can use on earth today, like controlling emissions. And that saving earth from apocalypse is the best possible practice we could have for fixing other planets. There is no magic "fix everything" button. Even with the right technology, you still have to do the work. Today, we have the right technology to save Earth, and what determines the continued survival of the human species is whether it is capable of doing the work.