I'm still not buying it. Impeachment didn't tell anyone anything about the Bidens they didn't already know. The idea that Joe would be cruising to the nomination if not for impeachment is ludicrous.
The fact that the polling shows vulnerability on the issue means that his opponents would have used it against him effectively anyway, and it would have been better for us if he hadn't collapsed so early, while Bloomberg and Klobuchar still have plenty of time to get their acts together.
I think you overestimate the attention span of the typical American voter. I don't suspect anyone outside hardcore political circles knew about the Ukraine and Burisma dealings in terms of the Bidens before they tried to use it against Trump.
That alleged phone call put the Bidens' corruption front and center for all to see, most importantly in a way that separated it from the "he said/she said" of daily political whining. By packaging it in a way that was intended to attack someone else, they inadvertently provided an even more potent cudgel against Biden.
Ah, those good ol' unintended consequences.