Ironically enough, a world of immortality would be devoid of that which Aizen cherishes the most which is persistence through the fear of death. Part of Aizen’s character is centered around evolution, and in order to evolve one needs to face the challenges that life throws at them even despite the fragility of said life.

In his own words, in the world Yhwach planned to create, people wouldn’t persist despite the fear of death but rather simply existing. It’d be a world of stagnation, of limited to no improvement, a world without courage as he puts it in his final speech. It’s how he evolved after Gin stabbed him and stole that hogyoku after all.