Whether you agree or not, there is genuine concern among committed anti-Trumpers that Biden's apparent frailty could influence 'undecideds' and and give Trump the presidency.
There's not really a great option available, though. He stays in, and there's a risk he loses. He steps aside and Harris runs? There's a significant risk she loses. There's a free-for-all at the convention and he's discarded in favor of some other candidate? Likely even a smaller chance of a win. The thing is that in any of these scenarios, the Democratic party will be divided. They've never been good at unifying behind a candidate, and at this late stage in the campaign, the chances are that whatever path forward they settle on will be beset with utter chaos.