One had to say that Aksai had chosen the right time for the evacuation of the regular citizens, allowing them to escape Devil's Den prematurely.
If the corrupted druid had been given sufficient time, he could have let the demon tree grow some more and extended the range of operability for his tree devils at the cost of facing temporary instability in the Yantra.
He could have then managed to stop a majority of the citizens from escaping Devil's Den.
However, Aksai's initiation of an all-out war made that possibility practically impossible.
Because Aksai had charged toward the city square with an army of martial artists, the tree devils from various danger zones also had to congregate in one region so that they could stop the intrusion.
The demon tree itself was busy handling the increased load of operating on loads of tree devils at the same time.
Strength and growth come only through continuous effort and struggle.