"if someone can play Elphaba they can play Marilyn"
That's bull. The two roles couldn't be more different. Compared to Marilyn, Elphaba is easy! Elphaba sings in 13 numbers, sometimes only a few lines. Marilyn sings 17 numbers most of while are solos or mostly lead by her. She opens Act 1 belting a song, she closes Act 1 belting a song, she opens Act 2 belting a song, she closes Act 2 belting a song.
The role would definitely need to be alternated or drastically changed.
"Charlotte, we're Jewish"