Some months back, I had expressed the belief that the reason the DNC pushed Biden for a second term was that they weren't sure how to deal with Harris, should she decide she wanted to be the candidate. If they tried to get her to step aside and she resisted, it would have looked bad for the party to try to sideline a female POC. But she did not impress as a VP (which is a truly low bar in US politics) and anyone could see that she wasn't up to the task.
But Biden's debate performance couldn't be ignored and he was finally made to end his campaign. The only option was Harris. Someone who lacks the ability to express confidence smoothly and who struggles to control any situation she is in. This, plus her frequent troubles expressing herself clearly, is a bad combination for a political campaign. I wasn't ready to write her off because Trump always seems to be walking a tightrope. But, just like 2016, polls could not figure him out.
Pollsters will weep with relief once he exits the political scene. Assuming they don't go extinct, first.