While you’re right that the democrats don’t want Biden as their 2020 nominee, I think that their biggest motive for the Ukraine hoax is to deflect attention from the Barr/Durham/IG investigations into the origins and perpetrators of the Russia/Trump collusion hoax.
Biden is not that important to the democrats, and the number of people that could be brought down by Barr/Durham/IG investigation, is infinitely more important to democrats. Democrats would be brought up on charges of sedition, perhaps treason too, and the people that could be brought up on charges would range from the former president to the high-ranking people in the DOJ and FBI and CIA and even in congress, and no doubt, many in the lett-wing media that assisted in perpetrating the attempted take-down of a duly elected president. Biden is but a mouse compared to the elephants that could be in very deep danger of prison time.
To continue with this line of reasoning, I think that the current furor not only deflects attention, but also puts the politicos from the countries being contacted by Barr and Guiliani on notice to not cooperate. Notice that apparently the President of the Ukraine made a statement this weekend that he did not want to get involved in American politics.