hmm i think that they assumed that they would be able to coast through this election Weekend at Bernie's-ing biden by emphasising trump's criminal charges, rape scandals, fearmongering about project 2025, etc. bad polling against trump was waved away by saying that it was republican biased (remember that isaac chotiner interview with that guy who ran "hopium chronicles" or whatever? that guy was a dem party insider, so to some extent i think that they've started believing their own propaganda like many republicans tend to). i think they thought that biden could outperform trump at the debates as well in terms of intelligibility but they overestimated his ability to orate at this stage and underestimated how "baked-in" trump's incoherence is to the wider electorate. think about how every vote blue no matter who guy was shrieking "the lugenpresse are focusing on "Biden's" "Dementia" while IGNORING how stupid trump sounds!!!" for a couple days there. trump's brain was 90% cocaine in 2016 and it is the same way today, but people were genuinely surprised by biden's inability to carry a debate. the donors that pay for nancy pelosi's stock market purchases got spooked and thought that biden was too cooked to outperform trump (probably not wrong given the polls) and started making calls so the plan had to change.