He won't do well with PoC and women once his "identity politics" comments and fixation with the white working class over other voting blocs becomes widely known.

He won't win Latinos when his 2007 immigration reform vote (and going on Lou Dobbs to talk about it) becomes widely known.

He won't win swing voters and moderates who dislike high taxes and will not vote for someone with a long history of far-left politics and praise for leftwing dictators (Castro, Noriega) and who honeymooned in the USSR and spoke glowingly of the country during the height of the cold war.

He also won't win over enough white working class votes to offset underperformance with the aforementioned groups. Trump solidly has these voters, who will simply vote for "the real thing" ie a populism that more strongly appeals to their racial resentments, instead of the generic class populism pitched by Sanders.