A Saint Spirit Vein could die for many reasons.
The sudden increase in the ambient concentration of aura in the Demon Realm could've been the reason. The slow and methodical absorption of its Qi by the two trees without returning anything could've been the reason. Or the constant spatial merging could've been the reason.
Whatever the reason was, the Spirit Vein had died, which meant it was no longer going to create any more Saint Qi for anyone to use.
It wasn't such a big loss overall in terms of what everyone was going to absorb. No one was in the Saint Realm in the first place, so the spirit veins weren't even being used for cultivation at all.
What it was being used for were the Timeless Palace and the cultivation chambers. Without the spirit vein, everyone was now going to be forced to cultivate without the chambers.
