"But... even the first Dragon, I guess, would never have thought that the twilight of this world would not only last for three hundred years, but also up to a thousand years!"
A thousand years of Spirit Tide depletion!
Without the support of the world, even long-lived species like the Giant Dragon can only meet the inevitable fate of death and decay.
Lifespan!
This is the real constraint!
"Legendary occupational persons undergo qualitative changes, sublimation, can prolong life... but still cannot match the giant dragons. Even if they are giant dragons, they may live for thousands of years during the resurgence of the Spirit Tide, but once the Twilight of the the Gods comes, relying solely on their own strength, I'm afraid they won't live up to a thousand years!"
"So even if the first Dragon made some arrangements, after a thousand years, they may not still be effective!"
Time is the most terrifying weapon.