My prediction

Within a year (prob after the election), Biden will publicly say he was diagnosed with a form of mental decline (parkinsons, had a stroke, etc). And then it will be revealed that this happened late 2023 or early 2024, and the democrats were hoping to prop him up tru the election, and that he would resign shortly thereafter. Then the debate, and things were no longer hidden.

I think this decision was made shortly after the debate, they were just waiting on exploring their options and timing.