“I don’t see why the wife wouldn’t have to follow the husband like any other wife.”
She has to follow him, but maybe he wants to or agrees to take on her minhagim.
“And I definitely don’t see why her minhagim should be the trendsetter.”
Why not if he doesn’t have a mesorah? As you wrote, if he has no mesorah, he can choose his minhagim, so wouldn’t the next best (to following his own mesorah) be following his wife’s (who does have a mesorah)? I don’t think that he has to if he doesn’t want to, but wouldn’t that be the logical choice?
After all, why should the wife have to change her mesora for no reason?
It’s one thing in an ordinary case where she is changing her mesora in order to take on her husband’s, and even that can be very difficult for some girls. But at least then there is a very good reason for it. But here where he has no mesora anyhow, why shouldn’t her husband allow her to keep hers? (unless of course he has a particular problem with her minhagim).