I'm not sure Biden is out. Taking Biden out will give some legitimacy to TRUMP calling out the corruption of not only Biden's involvement with Ukraine but also the entire corruption of the Biden/Obama administration and the illegal spying on TRUMP.
The media can try and prop up Warren all they want but there is no way she encourages the black vote to show up at the polls. And while Obama would do his part in campaigning, being on the stage with Warren will not have the same impact as him being on the stage with Biden.
Basically democrats continuing with impeachment is dependent on covering for Biden's corruption.