I had a chapter on Lincoln not being assassinated in Other Lives (2013). I accept that as a white British man, I might not be felt to have legitimacy in commenting on the period in the USA. However, in my mind, whatever Lincoln might have turned out to be, even if mediocre, he would have been far better for the USA than Johnson proved to be. Johnson betrayed a lot of what certainly the post-1863 Union fight had been for and really 'let off' the Confederate States, let alone their leaders.
Of course, Lincoln's survival would not have resolved all the challenges that the USA faced in the following decades, but it would have got it off to a better start and just perhaps would have reduced some of the racial discrimination and violence that followed. At least, it would have blocked the worst of the Confederates from gaining/retaining significant roles and perhaps may have reduced their freedom to introduce Jim Crow legislation and effectively disenfranchise many Black voters through the 20th Century and beyond.
No politician is without flaws. Perhaps this novel is shaped by recent events, notably the almost immediate attempt by some in Congress to impeach President Biden or those to the left of the Democrat Party who felt even Obama did not go far enough. It certainly does seem to highlight the tendency of Americans these days to prefer legal challenges over making a case at the ballot box, so I am coming to see it as less a book about Lincoln and more about 'what if 21st Century US political behaviour had begun 150+ years earlier?'