Definition of "momentousness"

There's a scene near the end in which our ensemble receives some wholesome wisdom from a woman selling balloons in the park, portrayed by Angela Lansbury. Her presence and the momentousness appended to it make no sense, because this role was very clearly conceived for Julie Andrews, who wanted no part in all this.

2018 December 12, Charles Bramesco, “A Spoonful of Nostalgia Helps the Calculated Mary Poppins Returns Go Down”, in The A.V. Club‎, archived from the original on 24 May 2019