I was thinking about that earlier in the day myself. While Tom Baker may have been the first choice for the role for a variety of reasons (oldest living Doctor, fan favourite) I imagine they have been prepared to go to one of the other classic Doctors. Ouch, really? I know Davison is Moffat's favourite, but I didn't realize he was so harsh towards C Baker and McCoy. I can easily picture any of them portraying the Curator perfectly.