Maybe another way of asking it is to suggest how big would Metallica been if they had been christened Anthrax, and vice versa. For example, I'm sure SOWN would have been a huge commercial success like the "black" album if it said Metallica on the cover.
The point I'm getting at is Metallica's huge "cross over" success amongst fans of other genres of music and metal. If I say to people I'm listening to Metallica on my Ipod, even ardent non metalheads give me a grudging "yeah, I've heard of them". If I say Anthrax (or Slayer or Megadeth) then they fall about laughing, saying "how can you listen to that pish" "fucking Anthrax, that must be murder" etc etc.
I think Metallica is a great band name, its metal without being stupid or scary to all music fans. And I think its played a huge part in their success.