I'd say keep it.
NANOG's archives at Merit are probably more stable than
many mailing list archives I've seen, but it's possible that
something will happen to it in the next 5-10 years that kills it.
Multiple copies of list archives aren't a bad thing,
at least for relatively low volume lists like NANOG.
Old archives aren't usually highly useful, but once in a while
they can provide good insight into the "why did they do that"
sorts of questions.
The archive at gmane.org munges @ charactes into " <at> "
in an attempt to reduce spider infestations,
and it's probably not a bad idea except when people are
trying to explain specific syntaxes that this garbles.