I am putting the odds at 60% that witnesses will not be called.
Cocaine Mitch is going to put forth a resolution that will call both Hunter Biden and Bolton with a single vote. If the Dems want Bolton then they will be forced to vote for Hunter as well. This puts the Dems in a no win situation.
The impeachment is already inflicting enough damage on Biden that now it will be more then likely a contested convention. Biden's polling numbers are going down compared to Bernie's. At this point in time, this is already a no win situation for the dems, but would still give them a fighting chance to maintain the house. Fact is, if Biden and Bernie both have around 40% of the vote, neither one can be voted for since picking either would split the party. On top of that, since all other candidates are polling in the low teens, none of them can be picked either. So the only option is to pick someone not currently running, which never works unless there is some outside major event taking place like the Great Depression. So, this path leads the Dems to loose the presidency but still gives them a 50/50 of maintaining the house.
If Hunter testifies, Joe Biden's candidacy is over and Bernie wins. Right now, 76% of the country identify as either conservative or centrist, and Bernie is so far to the left, even centrists need a telescope to see him. So Bernie has no path to 270. On top of that though, Bernie as the candidate puts the Dems between a rock and a hard place. Either support Bernie and loose all credibility, or denounce Bernie, which for all intents and purposes would be an endorsement for Trump. (FYI, Bernie's plans are so radical, that any answer to any question on whether you support those plans besides a clear no would be a suicidal political move.) I suspect most Dems would pick the former, and that, combined with voting to impeach, would cost many Dems their seats, allowing the Republicans to take back the house.
Not calling any witnesses would be the best play for the Dems, and, maybe if Obama endorses Biden, Biden could even win the primary fair and square. But if Hunter testifies, even an endorsement from Obama will not save Sleepy Joe.
With that being said, I would not be surprised if the Dems cut off their nose to spite their face and call Hunter to get Bolton.