Yhwach promised Renji, and Rukia that he would kill them in their most happiest moment in the future before been under KS by Aizen.

Renji is going to get killed in the Hell Arc?
Yhwach also said “all bankais” are destroyed in the future. He took that moment and use it for Renji’s bankai which was really Aizen under KS effects. Meaning Aizen’s bankai will be the only bankai that will be “safe” from been “broken” because Yhwach already remove that “event” from the future by bringing it to the present and since Yhwach mistook Aizen for Renji that means Aizen’s bankai was never broken and will be the only hope everyone has against the new enemies in Hell.
But how are “all bankai going to be destroyed” in the future and by who or what?
If you look at the final battle, Yhwach dies in the same spot he came out of the portal. He wasn’t given a chance to move away from that spot during the battle at all. Did Yhwach foresaw this and will pull a Momoshiki with Kazui?