Agreed but scale has to be a factor. Millions of American children are in the custody of grandparents or the foster system at any given time.

Secondly, there is way more friction here than with a term like guardian, which represents a situation that is universal thoughtout humanity, and always has been. The common language only changed because of the need for accuracy and inclusiveness in things like school handbooks, not because it wasn't fully understood and accepted by the population as being an inevitable element of human populations for anywhere from 1 in 10 to 1 in 100 children.