While I agree that Trump will be hard pressed to win in 2020, but that all depends on whom the Democrats nominate. That said, what does this poll really mean? Probably not much. Remember prior to the 2016 election and all the uproar about the never Trumpers. About all those Republicans who were going to vote for Clinton. Here's something to think about. 75% of republicans viewed Trump favorable the day before the 2016 election. 24% viewed him unfavorably. The talk was most of those 24% were never Trumpers, thus Hillary would win. Question 11.
https://d25d2506sfb94s.cloudfront.net/cumulus_uploads/document/l37rosbwjp/econTabReport_lv.pdf
Yet on election day, Trump won 88% of the GOP vote, Hillary 8% with 4% voting third party. This means at least half of those who had a negative view of Trump voted for him, another 17% who had a negative view Trump voted third voted third party instead of for Hillary. Hillary did garner a third of those who viewed Trump negatively or unfavorably. What's worse is that 30% of conservatives view Trump negatively on election day, 69% of conservatives view him favorably. On election Day Trump received 81% of the conservative vote, Clinton 16%. Hillary received only half of all conservatives who viewed Trump negatively, unfavorable.
https://www.cnn.com/election/2016/results/exit-polls
Your poll comes close to the day before election day poll. But in the end, Trump did receive around half of all those Republicans and conservative votes who had an unfavorable opinion or negative view of him. Why? I think it goes back to: My candidate sucks, but he doesn't suck as bad as yours. Or that no matter how bad my party's candidates is, he is still better than yours.
If you go back to Question 10, the first link, you'll see 88% of Republicans had an unfavorable view of Hillary Clinton. That is the exact percentage of the Republican vote Trump received. We many not like Trump, but we dislike Hillary more or as Republicans we dislike our GOP candidate, but we really, really disliked the Democrat candidate more. More telling is that 7% of Republicans viewed Hillary favorably, she received 8% of the Republican vote. 7% of Democrats viewed Trump favorably, he also received 8% of the democratic vote.
Bottom line, I wouldn't read too much into your poll. Probably half of those who say they won't vote for Trump will. A quarter probably would vote third party if they don't vote for Trump before they would vote Democratic which in reality doesn't leave you with that many actually voting for your candidate.