Patent expired. Plus, you shouldn't be able to patent a length of something.
Although, the US patent office has allowed people to patent peanutbutter and jelley, so who knows?
The fact is, that Meshuggah is not the first artist to use an eight string guitar. There are dozens of artists who used them before- for classical, jazz, and for rock and roll. The seven string guitar will never make it's way completely into mainstream, because people fell in love with six strings for no reason whatsoever.
Never say "never." Oops, I just said it.