I don't believe that anyone really minds them being the Exeter Chiefs, I'm sure there's plenty wanting a change of name, but the word chief is not solely used to describe indigenous Americans.
The problem is the imagery that goes along with it.
Exeter chiefs is fine. Just go for a different image, a naval chief, a Celtic chief since that is their heritage etc. Just don't use a load of imagery that has bugger all to do with the club, it's roots, or anything whatsoever, and is offensive to the people who's heritage it is.