What's more intresting is the Ocean Palace disaster, actually. Destruction rained from the heavens, and yet the world survived... I'd wager that it's due to Lavos not gaining enough power...
Ah, and Magus, in reply to you: Yes, parasites don't kill hosts. And indeed, nothing has never-ending energy. However, due to the fact the Planet doesn't have eternal energy, Lavos coulden't use it anymore. I think the attacks that destroyed Zeal and eventually, the whole Planet, were just him giving a big loud yawn. Not to say he didn't want to destroy the Planet, but when you fight him in 1999 A.D., he's weaker then usual, due to his long sleep. Not unlike how the FF X party attacked Sin by making him docile with the Hymn of the Fayth, Lavos was docile (Or sleepy, if such a term applies for Lavos) due to his sleep of 13,999 years, and his longer sleep, which took 65,001,999 years.