If Biden had not withdrawn from the race, Trump would have had an almost 100% chance of winning. In other words, Biden had virtually no chance of beating Trump. Therefore, after Biden himself announced the withdrawal of his candidacy for the presidential election, the Democrats had a good chance to beat Trump. This could well be done by the current US Vice President Kamala Harris, in whose support Biden himself also spoke out. There are still a little more than a hundred days left before the election, and this should still be enough time, because Harris will not have to start from scratch. It will be supported with all available resources by the Biden campaign headquarters and many influential politicians.
Therefore, the fact that Biden withdrew from the election race did not increase Trump’s chance of victory, but, on the contrary, decreased it. Even before Biden dropped out of the race, Harris was only a few percentage points behind Trump in popularity. But Harris is a woman and a black woman. There may be surprises here, both in a positive and negative direction for her. But nevertheless, she has already announced that she will nominate herself as a candidate in these elections.
While I agree that the sentence "Biden withdrew" may increase the chance of Trumps loss, if that in return becomes "but Kamala is the replacement" then Trump will win anyway. Not because Biden is better than Kamala, I would say Kamala is better, but at the same time there are more people who hate Kamala than people who hate Biden.
The only reason why Biden was about to lose, was because of his old age and the questions regarding his decline, and to be fair Cognitive decline is a serious issue if you are willing to give 4 years of a nations presidency to someone. This means that Kamala is much better at that, but Biden wasn't hated that much, sure disliked maybe, but not hated, whereas Kamala is hated by some people.