I've seen a number of communities that are otherwise dead without Reddit reposts, and being the most subscribed community for a given topic with the latest post being months ago is definitely not going to attract new users.
It's either don't repost, and new users won't join because of dead community, or repost and have some activity, and maybe new users will join. With dead communities, new users won't magically join, and new content won't magically get created.
One such example was the bcpcsalescanada community, which was revived due to reposts.