I personally think Harry and Hermione wouldn’t work at all as a couple. Their personalities don’t mesh well (being so similar), and it’s repeatedly made clear that they both have no idea what the other needs emotionally.
Also, there’s the fact that neither of them show any romantic interest in each-other throughout all seven books. But old Stevie presumably ignored that little detail.
Personally, I think JK Rowling was influenced by Steve Kloves’ writing for the films, and (as a result) she started to doubt her own writing and wonder whether it would have made more sense to have something happen between Harry and Hermione. Steve Kloves got what he was aiming for; making Rowling doubt herself over her own writing, and think maybe Harry and Hermione made more sense.
It’s quite ridiculous when you think about it. Harry and Hermione show no romantic interest in each-other, but Steve Kloves’ meddling was enough to make Rowling toy with the idea of something happening. Over a decade later, we’re still trying to clear up the mess he made in the fandom.
Thankfully, Rowling never added that in, but it would have been a complete 180 turn on all the series before that.
Seriously, Steve Kloves can go do one.