We live in unprecedented times.
The last time we had a sitting president not seek reelection like this was LBJ and he announced his intentions in March, 1968 [1]. We don't have the communications media we have now. This had a massive impact. The Democrats were largely unprepared, in part because RFK was assassinated in June.
This ushered in Nixon who expanded the war in Vietnam, started the illegal bombing of Cambodia that killed hundreds of thousands [2], started the War on Drugs [3], reshaped the Supreme Court and ultimately created a crisis leading to his resignation in 1974.
Biden should've announced last year he wasn't seeking reelection, giving time for a real primary process. But we are where we are.
Some will point to Biden's achievements, and there definitely are some major achievements, but elections are forward-looking. It's not just about how Biden is now. It's how we would've expetged him to be 1 year from now, 2 years from now, 4 years from now. And those prospects weren't good.
I do believe this was the right decision (albeit way too late). This is an opportunity to reset what otherwise was looking like a bleak race for so many of us.
[1]: https://www.history.com/news/lbj-exit-1968-presidential-race
[2]: https://www.history.com/news/nixon-war-powers-act-vietnam-wa...
[3]: https://www.britannica.com/topic/war-on-drugs