AkiraKonoe

In DxD, humans aren't born with the raw power of high-class demons and a natural talent that makes their strength increase effortlessly. They must acquire it through training, contracts, or magic. The current problem is that a talented human (e.g., a powerful exorcist) begins to master complex techniques and gain decent combat experience around age 30 or 40, then their body declines and they die. Humanity is constantly starting over. If a human lives for centuries while remaining young, they can accumulate, refine, and directly pass on centuries of martial and magical knowledge. This creates an "aristocracy of human fighters" capable of competing with demons without changing their race. This is even more important in the case of Sacred Gears (Balance Breaker). Achieving Balance Breaker requires an amount of experience, stress, and emotional maturity that most humans don't have time to attain before dying or being killed. More time means an exponentially higher probability for Sacred Gear wielders to achieve the Balance Breaker. Instead of having one or two awakened humans per generation (like Issei or Vali), humanity would have a standing army of Sacred Gear wielders at their peak power. The same applies to mages: human researchers or sorcerers who live for 2000 years working together can push magical research to a level equivalent to that of a demon genius like Ajuka Beelzebub (who created the Evil Pieces system). Human technological and magical progress would explode, because the greatest minds would no longer die of old age in the middle of their research. In the modern world, the elderly might just be annoying and decrepit, but in a magical world? I doubt it...

When you live longer, experience and wisdom increase more naturally, and you tend to become stronger and more competent. Therefore, extending human's lifespan is a very good way to boost the average human strength level, while simultaneously reducing the number of humans with a high potential and strength to be convinced to become another race for an extended lifespan, like the Sacred Gear wielders joining the demons.

that he can one-shot Riser's entire team before beating him up

The MC didn't play the game? How could he not have understood where he was?

the ancestor of the legendary blind douluo

Seriously, an S rank? She's better than MC even without meta knowledge about the lore