I think you greatly under-value the cumulative effect of the Steele Dossier/Flynn/Manafort/Russia-gate/impeachment. Orange One ultimately survived, but the scandals that led to impeachment broadsided the administration and bound them up in ugly legal scuffles that only got uglier as time went on.
Bolton's book is just another shot at the same target - the big orange beast, but the beast bull is full of arrows now and starting to wobble. The book is probably unnecessary, but it's hitting the target in a way the Steele Dossier didn't. Bolton may not run The Heritage Foundation, but he's connected and I have to feel that if the conservative base was truly all in on Orange One for another 4 years this book would've never come out. Bolton could've been appeased. There are fractures running all through the GOP.