Not to take away from most of the basic points you are making, but more people would not have had the chance to see Brooke Shields in Wonderful Town than saw Donna Murphy. Previews started on November 5, 20003. Murphy played her last performance on August 28 or 29, 2004, if what I find online is correct (and it is at least close to correct), four weeks before her contract was over, leaving early. During the time she did play in the show, I think she missed around 60-70 performances because I think it was said that she had missed 100 altogether, but that was including the last weeks when she was completely gone. If so, that would mean she played around 260-270 performances in the role. Shields took over on September 28, 2004, and the production closed on January 30, 2005. If she missed no performances, she would have played 144 performances, barring the possibility that the show played nine performances for a week or two around Christmas-New Year's.
I saw both of them in the role. I saw Murphy during previews, before her vocal problems started. I thought neither of them was altogether ideal in the role, but Shields was probably a bit better, despite being miscast physically.
Alexis Smith, a similar physical type to Shields, played the role in stock in 1957.