"Welcome To The World, Elpízo"
When the legendary beasts' spot was empty, there would be no one to replace the dead legendary beast until one of the heirs successfully became a legendary beast.
Most legendary beasts live past a thousand years, and could even live longer than that. Naturally, it was harder to become one.
In this era, the number of living legendary beasts had dwindled to the point of almost extinction.
Many sacred beasts with a chance to become a legendary beast still couldn't advance.
And when they exhausted their lifespan, they automatically passed down their bloodline to a new heir.
Then, the cycle started again from zero.
No wonder the phoenix mother disliked Blaze for being a mutant, because the chance to become a legendary beast was way lower for a mutant variant.
Especially when Blaze was a water phoenix who should be affiliated with blazing fire.