I think a 'relic' is specifically something that's no longer being used or has any relevance - an artifact, something you'd see in a museum; interesting, but with no practical use now that the world has moved on. Without Thor, Mjolnir has no use, since (in theory, anyway) it can't be used by anyone else - with him in the picture, it is no longer a relic, but instead a useful, practical weapon.