One could argue the chaos and uncertainty from upending his currently functional administration is not worth the upsides, while allowing for a natural transition via the standard election and end of term process is worth it.
The important consideration is that he’s unfit to serve for four and a half more years, and also that he’s unfit to win the election. In contrast, he’s probably not considerably more unfit for the next six months than for the past six.
He’s mildly unfit to serve now but there’s reasonable doubt he’ll be at all fit in a few years. That would be what the more rational people should be thinking. It’s not about day one it’s about year two, etc.
If he's unfit to serve 6 months from now, then how is he fit to serve now?
He didn't endorse Kamala in his letter. We'll see what comes out of the spin cycle this week.
e: looks like Biden endorsed Kamala according to other publications