Of course they are remembered for playing those roles, because they are the ones who played them in the film (a much more accessible medium than the theatre). Now I can't comment on Mary Poppins or Wizard of Oz because I've never seen them on stage but I would never go to see one of them expecting to see Judy or Julie on stage though, as I know I would be disappointed. I would be totally prepared to see their performances altered and even bettered on stage. I think it would be a shame that everytime someone gave a 'definitive' performance, that no one would dare recreate it.
I would say Whoopi is too old to recreate Deloris again, so she did the right thing and let someone else take on the role. In doing so, she let the songs have so much more range than she would ever be able to sing.
I think the fact they changed the songs and the era when it was set actually makes it easier to accept them as different performances. If Patina was singing Whoopi's part in 'I Will Follow Him' I'm sure I would be disappointed. Disappointed because she didn't get to show off her voice more.
I'm sorry you didn't like it, because I love it and it always cheers me up, but if we all liked the same things, the World would be a very dull place.
Once you start to spread it, Baby, if you let it, Love comes right on back to you . . .