The cost of defeat in a Civilizational war is oblivion. Once you are defeated and converted, the next generation doesn't identify with the defeated civilization/tribe and then nobody is left to fight for it. It's forgotten, you get no do over. Ex: Zoroastrians of Persia

The reasons Hindus still exist are:
1- Demography: We had the numbers on our side n they couldn't convert all of us
2- Courageous ancestors: Even the most down trodden of our ancestors never gave up. Chose to pay Jiziya but did not convert out
3- Warriors: people who fought back

Nobody seeks revenge for the sufferings of Persians at the hands of invading turaqas. It's all done n dusted. It's all forgotten.

Once you loose, no one will even remember that you lost or suffered. Hence it's important to destr0y your civilizational enemy completely. No Mercy

For those who get the reference, our en3mies only deserve the Shinra Tensei.