Nobody is going to waste their time in making 51 attack on a new coin or an unprofitable coins. Bitcoin and ethereum is no longer susceptible in being attacked anymore due to large hash rate. New coins need time to be accepted. So if a unvalued coin is attacked, people will soon abandon it.
You are right, such a waste electricity and time for doing that because new coin doesn't have much value.
It would be a different story if anyone can put the 51% hash rate power on a single miner in Bitcoin or Ethereum network.
I think every new coin can't prevent 51% because the miners are still very few, anyone could have above 50% network hashrate at the early launch.