I agree, but, honestly I have thought and thought about it, and since they were attempting to make the movie with more of a "like the books" attitude I don't think that Julie minds at all that she isn't in it. It seems to me that it would be awfully degrading to go from the absolute lead to a 3rd or 4th level below a little to much to ask. Van D yke was a secondary lead as Bert, but, just a comedy relief as the bank patriarch. The exact status as his original portrayal of the bank patriarch. No change in status for him. I remember when they announced Star Wars... The Force Awakens. In that one both Harrison Ford and Carrie Fisher returned as the same, older, but, able characters. That worked, I don't see how the same thing would have worked in Mary Poppins if for no other reason then the length of the franchise and their participation in them was so much longer and they were both older then the Star Wars Movies. I have to wonder if D. VanD would have gone in for any other character then Bert or the Banker.
I'm not trying to change your mind, just to try and explain more deeply, the reasons why I'm happier that she didn't, then I would have been if she had. The Fairy Tale nature of the movie would have been compromised for me and I would have been more focused on the personalities then the story.