The more useful thing to users would be a list of alternatives for each closed subreddit. No reddit alternative is going to grow to anything useful from this.
Why not? Typically users of reddit are subscribed to more than one subreddit, so migration to a single destination that can handle all of those communities seems like an obvious choice. The network effect of many users advocating that their second favourite community migrate over to the "new reddit" seems reasonable once people start to establish a userbase there.