The impeachment has become all about the Senate elections in November.
It is not about Trump. Trump WILL NOT be put out of office as that would take 67 votes (47 Dems/Indpendents plus 20 GOP) and there simply are NOT 20 GOP senators that will cross the aisle to vote against Trump as that would be political suicide.
However, in November, 33 senators are up for re-election and 19 of these are GOP senators with many in closely contested seats. The strategy on both sides is to gain (or at least not lose) any seats in the senate.
Will a vote for or a vote against bringing in witnesses do the most damage these senators? A vote to call in witnesses will definitely cost them some of the GOP base that they need for re-election. But a vote to "blindly" stick by Trump may cost them needed votes of independents. Be assured that the pollsters are working 24/7 to assess the answers to those questions.
Personally, I think the witnesses run the risk of inflicting more damage to the GOP cause than the risk of refusing to listen to them, thus I suspect the leadership will design a strategic motion to move this BS trial forward without calling Bolton or anyone else.