Replacing the major candidate who already passed significant democratic legislation for 4 years is unlikely to result in greater Democratic faith or voter galvanization.
Harris certainly does not have that kind of momentum or support.
Any sort of candidate scramble now is all but forfeiting the 2024 election to trump.
Democrats already functionally control the Senate and have for biden's four presidential years; I can't see how nervously replacing Biden wins them any more seats, even optimistically.
That said, by the numbers, keeping the house majority and packing the court is more likely than a complete constitutional rewrite, a strategy I used as an example to show just how bleak the chance of restoring the us balance of executive power is.