If you want to terraform Mars, it's probably not because you decided Earth was beyond salvation; I haven't heard many people wanting to abandon Earth. Most people in favor of terraforming Mars want to have a 'backup' for intelligent life, or want to begin humanity's expansion past the bounds of Earth's atmosphere. The 'backup' motive is comparable to suggesting someone backup their electronic data instead of just trying to get very reliable drives.

I am also not sure that it is possible to 'terraform' Earth. 'Terraform' means 'to make like Earth', and Earth is as Earth-like as is possible, no matter how hot or cold it is.