Yes, because a) Gaea was controlling who could come back from the dead, so her army was endless but she could keep ヒーローズ from coming back, and b) people would still be being born, and the earth only has so much room for people... it would only be a matter of time before resources ran out, and people'd be living in squished quarters.
Yeah I think it was essential that they rescued Thantos when they did, because the world would quickly become so vastly overpopulated that living would actually be painful. We'd have to stop giving birth, and because no-one could die then all the 動物 and plants (do they go to the アンダーワールド too?) would also keep reproducing, and everyone would be horribly squashed.