I've seen similar questions, but not specifically in reference to new altcoins.
When a new cryptocurrency is released, the developer(s) have limited amounts of resources. What prevents a group from carrying out a 51% attack while the mining network is still small?
I realize there isn't likely any benefit to the group carrying out the attack, since coin value will subsequently be lost, so the motivation would probably be carnage or the desire of eliminating a rival coin.