This one is pretty surprising, Duncan kind of seemed like the typical Christie/Haley/Pence etc Never Trumper thing and acquiesce and do the whole 'I won't vote for Trump but won't vote for Biden either!' So for him to actually state he'll vote for Biden is pretty huge.
It also could be the start of more Republicans coming around to this view. There's definitely been a few hot takes of 'Where are all the Republicans for Biden' this time around from some, so I imagine people like Liz Cheney may follow suit.
He's strategic. He knows four more years of Biden will be better for Republicans in the long run. Trump coming back int power probably hollows out the Republican Party's bank account and infrastructure even further, forces them to accept even more of Trump's transgressions, and limits their ability to win important midterm and off year races, thereby shrinking their bench.
In turn, I think a second Biden term with Trump out of the picture gives Republicans more of a chance to regroup, rebrand, and have a better 2026/28. Even as a Dem, I'd be ok with that if it means Trump is gone for good.
Georgia is a great example. A Biden midterm in 2026 probably sets them up to hold the governor's mansion and beat Ossoff. But a Trump midterm probably makes those races more of a reach for them, even with good candidates.
This is what a lot of people don't understand. For all the talk of dictatorship, Donald Trump is almost certainly not going to be alive by 2028, and the VP is going to be hated by Trump supporters by then like everyone else he's ever worked with. The sh-tshow that is a Trump term is not going to go well for Republicans on top of all the other problems America is facing.
