I really think this would be a bad move if it was motivated by this, and not Biden's complete incapacity for office. I expect the address to be a reiteration to his commitment to seeing out his Presidential term, a reiteration of his endorsement of Harris, and a "proof of life" style address given he hasn't been seen for a week.Biden to make address at 8pm ET tomorrow (1am Thursday UK) - I reckon he’ll immediately step down so Kamala Harris takes over and shows the US what they can have instead of Trump. It’s a perfect plan for the Democrats to get their candidate elected.
Harris will be better off campaigning as a new start, not as an incumbent - despite being the existing VP. She's had a low profile in office.
There's also nothing between now and November to "show the US what they can have" - we are now in the election period with active campaigning and she will have no governmental agenda to deliver
As @jfollows points out, it is what Republicans want, too.