That poster has a point. I’m a Brit here, so no skin in the game. You’re more focussed on being right that trying to comprehend that people look at things differently.
No one thought Biden was good, but he was the candidate and most know that name recognition is key in US elections. Most presential candidates fail on their first run. Kamala, despite having some OK polling numbers still has to get through to disengaged American voters who do not follow politics and probably know little more than the attack lines heropponents will throw at her this campaign. They have to define her before others do. This option is riskier than you realise, the only thing that changed was Biden became a riskier option than before.
Things are less black and white than you want them to be. Nuance and grey area is key, despite being inconvenient.