I think that tourists were a big enough part of the Broadway audience by 1980 that a replacement who might bring them in would have made a difference. I agree that the chances of Julie Andrews taking over were close to nil (except I'd get rid of the "close to").
And it was my perception that Sweeney probably had more repeat visitors than most long-running Broadway shows of the period. although not enough to allow the replacement leads more than four months.
