It's not about trying to convince his hardcore base, they are far gone. What this is for is to convince the independence and the Republicans with a conscious that may vote Democrat just so that they can try to right the boat back into the right direction.
And if he's pissing off the military, they may in fact vote Democrat next year. As long as the nominee isn't doing anything stupid or illegal, they should be able to beat Trump next year. This is why I don't support Biden as the nominee, I don't know how many skeletons he have in his closet or if he'll say something stupid to a minority base and pissed them off.