Stalemate Breaker.

Some of the Supreme Origin gods have used their Supreme Laws several times, and some of them have even died. That's many Origin cycles worth of Origin essence production that they were saving for the era of conquest gone to waste in a fight of a few months.

After all, not many Supreme Origin gods have world fragments to help them bear the burden of their Supreme Law. In fact, there are more Supreme Origin gods than Origin gods with world fragments. 

So even if there were no era of conquest to make them give up on Legion temporarily, it is not certain that they will be able to afford the fight if it continues for ten thousand years. 

The only thing that is certain is that they won't give up. Legion is also determined never to capitulate. This fight will continue for the rest of the era of conquest, if it must. They are determined not to be the ones to break first.