Because while anticipating the future, he knew clearly that the future was the result of his neglect. Otherwise, with his authority as Star Lord, how could he allow the enemy's power to grow stronger and stronger under his watch?
If he wished, he could completely nip the bad omens in the bud before they even emerged.
"Accomplish it in one battle!"
"Determine Heaven and Earth in one fight!"
Jiang Chen knew his thought process was to accomplish it in one battle, to eliminate all enemies at once and thereby fulfill his grand plan.
If he didn't let the enemy loose, how would he expose the unstable elements hidden within the Human Race? The Three Emperors and Five Sovereigns, for this plan, did not hesitate to confine themselves in Fire Cloud Cave; the Human Race, for this plan, did not hesitate to walk from the peak toward decline, allowing themselves to be bullied.