There IS an overwhelming need to stop Trump. If people are forced to pick ONE reason many who "like" Biden would pick that alternative -- it does NOT mean they are not happy with Biden. Other polls -- which are somewhat push polls - ask whether you think Biden, who is in his 89s should run for a second term.
However, with this poll, I think the majority of people here - politically engaged Democrats - would agree that stopping Trump to preserve democracy is a very important reason to vote for Biden. This angered me more in 2004 where some prominent Democratic pundits, including Carville and Begala, pushed the argument that people were more anti Bush than pro Kerry. I could not have been more pro Kerry, but I was anti Bush from 2001 onward (First the tax cuts, then the wars). Given Kerry's convincing win of the primaries one could make the argument that he had more personal support than their candidate who did not capture the nomination in 1992 until June.
Why go over ancient history? Compare 1992, where Democrats rallied around Clinton - with no suggestions I can remember that he should be supported JUST because he was the Democratic nominee or that people were just anti Bush and anti Perot. Unlike either earlier Democrat, Biden is an incumbent and he actually has a record that ANY first term President would envy.
Polls and articles like this diminish the candidate personally. One counter to it is that the few times someone other than Biden was polled against Trump, they did worse not better. This would suggest that Biden IS the strongest candidate as well as the de facto nominee.