it’s less the current polling and more the “candidate cannot complete a sentence, let alone make an argument about why to elect him”
everyone supporting JB before Thursday despite the polls assumed he would be able to campaign and grow his support
the debate showed that’s a not a fair assumption
Here's a thread with the president's post debate appearances. You're entitled to your own opinion, but it seems rash to say this if you've not at least looked at what's happened since then.