It doesn't need to be a "blockbuster". The budget was fairly low.
To your point about teens though, that's why they brought on Lin. He appeals mainly to 15-25 year olds, roughly. And Mary Poppins is a movie that specifically appealed to kids to begin with. Not sure why you'd think this would be different. Parents will be looking for something to see at the movies over the holidays with their kids and that will be it.
Ehh, its a stacked weekend before Xmas day. You have Bumblebee, Aquaman, and James Cameron Alita: Battle Angel all coming out right before Mary Poppins which gets it a Xmas day release. The alternative programming instead of Poppins is quite stacked.
I think Disney wants it to have long legs though.