You gotta be kidding me!
You want see large, challenging roles, just think (off the top of my head) about Rose in Gypsy (late 50s), Eve / Princess Barberra (sic), Passionella in The Apple Tree (mid-60s), Ella Peterson in Bells are Ringing (mid-50s), Coco (early 70s), Cyrano (late 70s, I think), Harold Hill (late 50s), Tevye (mid-60s), Fanny Brice (mid-60s), Peter Allen (2003), Mame (mid-60s), Evita (early 80s), Sweeney Todd (1979), Agnes and Michael in I Do! I Do (mid 60s) (although Martin and Preston did begin to play only 6 performances a week (mind you, 10 months after opening night), and their replacements for matinees were stars), Jean Val Jean (mid-80s). I could go on and on, and these are just musicals. There are a huge number of plays with massive, challenging roles for their leads (Tennessee Williams and Eugene O'Neil alone...).
I think there are a lot of people today who just don't see Broadway as their destination; rather, it is just a stop on the way to movies or a long running TV show. I am probably being crotchety, but I am just tired of all the times that leads have missed performances I attended in the last decade, most recently in the case of Frozen, which I saw in the spring shortly after it opened; both leads were out, yet no one else in the entire cast was also out, so I am not guessing there was not a flu going through the cast (it was not flu season).
I may be a cynic, but I left the theatre that night wondering whether they just were not in the mood that night; but also, luckily, I ended up not missing them a bit, because both understudies were up to the challenges of the roles (and the roles were so...characterless, that I would be shocked if ANYONE could have given a great performance).
PS -- there is NO role in Wicked that is as large and challenging as any of the roles above, although Elphaba may come close due to the vocal demands, if not size, of the role; and, to answer your question, Mary Martin would've been terrible in Wicked, not because the roles were too challenging for her, but because neither role was right for her. She was never IMO bubbly (Glinda), or serious enough for Elphaba. The few times I saw her, I got the impression that she was mainly playing herself, abetted by a warmth that drew the audience in and a pleasant, if unexceptional, voice. But, the show would have gone on!