I don't really care if the Republicans refuse to allow Bolton's testimony. It will make what is already obviously a charade more obviously one. It will give Democrats more to run on in November, and makes Trump's being voted out and a Democrat Senate majority more likely after the election. They already know Trump is guilty, and they don't care. It remains to be seen whether the American people care enough to get rid of these craven Trump stooges.
So what matters in the long run is not what happens in the Senate but how more damaging information continues to come out in the media between now and the election. The impeachment has had value only insofar as it puts Trumps malfeasance more in the public spotlight. The senate's inevitable acquittal is not important if the public continues to become increasingly aware of how corrupt Trump is and how coopted by his corruption Republicans have become.
Bolton's testimony won't add more to what we already know, and it won't change the basic dynamic moving toward a party-line acquittal. It will be astonishing to me if they won't go through the motions of allowing witnesses for political appearance's sake. The GOP's refusal of witnesses gives me hope that their obsession with winning every little battle with brute force might eventually lead to their long-term losing the war come November.
But the bottom line is that the witness question is lose/lose for Republicans. If Bolton and others do testify, that prolongs things in ways that keeps Trump's malfeasance and Senate complicity with it continuously in the public eye. The significance of the Senate Republicans acquitting Trump will have more significance in November than it will have in the short run. More terrible stuff will continue to come out about Trump and their vote for acquittal will be more difficult to defend later than sooner.