It has always been a foregone conclusion that the GOP-controlled Senate would never muster the 67 votes needed to convict Trump in his impeachment trial. Any testimony from John Bolton or anybody else no matter how compelling would not make any difference to the trial's outcome.
Trump may slip past a rigged jury in the GOP Senate
Trump will have a tougher trial in the court of public opinion when we, the American voters, decide Trump's fate on the first Tuesday in November about eight months from now. While the Republican senators don't want to hear any potentially damaging testimony from Bolton, American voters are more fair-minded and curious. John Bolton's book is already a best-seller based on pre-orders. Whatever Bolton has witnessed will come out before the election. His book will be discussed from now until the election. Bolton will be interviewed. People like John Kelly have already come out and said they believe Bolton's account of his interactions with Trump regarding Ukraine, which is tantamount to saying he doesn't believe Trump's account. Trump loses in a credibility contest to practically anybody besides Joe Isuzu (see video below).
Trump and White House will guarantee the success of Bolton's book
The White House has issued a formal threat to former national security adviser John Bolton to keep him from publishing his book, "The Room Where It Happened: A White House Memoir," sources familiar with the matter tell CNN. Whenever a book or movie are banned, public interest intensifies.
Trump has attacked Bolton via Twitter (Trump's weapon of choice). Trump has catapulted other tell-all books into best-sellerdom by tweeting his displeasure about them.
75% of Americans want to hear from witnesses
The incriminating nature of whatever Bolton reveals, (some has come out already and I won't be surprised if others in the Trump orbit make similar revelations) will serve as a reminder that the Republicans ran a sham trial in which evidence and witnesses were forbidden. Those Republicans won't look good to the 75% of Americans who wanted the hear from witnesses in Trump's Senate trial.
Newsweek reported that "the latest Quinnipiac University poll, released earlier today, shows that a large majority—75 percent—of voters think the Senate should allow witness testimony in the ongoing impeachment trial. Respondents appeared to have answered along party lines, with 49 percent of self-identified Republicans, 95 percent of Democrats and 75 percent of independents saying there should be witnesses."