Skaar can destroy planets too, any Old power user is at the very least to be considered as potent as any herald possessing the power cosmic. Caiera was able to feed Galactus so well that had he not been interrupted in his nap, he would also have it was on panel that he would have slept for for an a very long time.
Skaar can take anything Terrax can put out there without the axe, but even that isn't certain. If he could take that axe from Terrax, Skaar's chances of dominating would rise to critical stomp levels.
Some of that made no since, sorry what I meant to say was that Galactus would have slept for a very long time juiced up on the Old Power that Caiera fed him, in my opinion it makes the old power pretty potent stuff.