Well let's start by maybe trying a better name than Mr. Zip
We know for sure people will read the Flash if Wally stepped into it because he's the only one we got to see in the role for a long time. You have no guarantee they'd do the same for Batman or Wonder Woman. All the Batman replacements were always placeholders with "temporary" stamped on them. The Wonder Woman fanbase is one of the most difficult to please in all of comics (not always without reason), they are not necessarily going to embrace Donna the way Wally was embraced.
It very well could work. It very well could not. Neither Batman or Wonder Woman's stars have fallen enough the way Barry's did to warrant doing it.
They can continue the line with those characters, they just don't have to tell them in the present day. Do like kjn said and embrace a DCU that tells stories from different eras. We can see what a world after Diana looks like, while we see more of her adventures in the younger eras. Her title doesn't have to take place in the same period as adult Donna.
We've never actually gotten Wonder Woman's story to definitively end while an adult Donna continues on as a superhero to forge her own path. Wonder Woman has always been around. DC isn't likely to try either of our scenarios, so it's kind of moot, but narratively Donna could still come into her own without becoming Wonder Woman.
And why should the Wonder Woman mantle be passed down to Donna? What does Diana and her fans gain from this? If Donna can't make it without the mantle...sucks to be her, I guess. It's not Diana's problem.
Why is passing the mantles down the only way to allow character progression? That's strikes me as awfully limited. Really, there's nothing else that can be done to provide character development?