There is no doubt Biden has neurological and cognitive issues and is not fit enough to be President. But he had early onset of dementia in 2020 when he ran against Trump the first time. That's why he campaigned from his basement. His family, his staff , the leadership of the Democratic party along with the Main Stream Media have hid Biden's medical problems from the American public for years. Biden has only ever been President in name only, God knows who has actually been running things?
Will he be replaced? Who knows. It's practically impossible to replace him if he refuses to go. This morning Biden called into MSNBC’s "Morning Joe" and defiantly proclaimed that he would be staying in the 2024 presidential race. "The bottom line here is that we’re not going anywhere. I am not going anywhere," Biden said.
If the delegates to the Democratic Convention in August turn against Biden and refuse to vote for his nomination, thereby throwing it into an open convention they could replace him . That won't be easy, these delegates have all been hand picked by the Biden camp, they are Biden loyalists. In most States when a State holds a primary and a candidate wins delegates they present a list of people they have personally chosen to be delegates. The candidate knows these people, he's worked with them before and likely is owned favours by these delegates. It will take a lot to stab him in the back at the convention.
The major problem with replacing Biden is Kamala Harris, she's polling worse than Biden as are all possible replacements other than Michelle Obama. If they make a change and its not Harris, they run into a problem with the campaign funds they have raised , because the money was raised for the Biden/Harris reelection bid and can't be easily transfer to another set of candidates. Also dumping a Black Woman and replacing her with a White Man or even a White Female will cause a bloodbath among Democrats. It would certainly lose them the election, which looks like they are going to lose anyway.