If Metallica released a country album I (along with lots of others) would be curious enough to check it out.
Metallica fans would run the other way.
Do you want new fans? What do you want the new fans to be fans of?
What about the old (loyal) fans? Are they worth losing in order to gain new ones?
If you lose the old fan but don’t gain the new one, are you still glad you took the risk?