It would probably be easier to specify that "significant doubt" means
raising some specific fact and source for it (even if falling short of a
complete proof). For instance, a bad reason for deletion is "I don't see
a proof this is <1923"; a better one is "I think this photo is from 19xy
or later so the copyright tag Z may not apply".
The reason for this is that the significant doubt must be debatable,
otherwise it's just a variant of the Russel teapot.
Nemo