That was purely to provide an everlasting (and idempotent) solution. 
You could simply purge 1...n-1 and be fine. There are very, very few reasons (e.g. concern for key compromise combined with inability to timely reissue) for keeping old, duplicate certificates regardless of whether they are expired. Frankly, barring a well-justified reason, I advocate deleting all old, duplicate certificates every time a new, duplicate certificate is acquired. That has always been the behavior of CertSage, the ACME client that I authored and maintain. If certbot were to follow this pattern, there would be no need for the symbolic links at all and anything using the certificates certbot acquires could simply reference the actual files since they would no longer have changing numbers appended that cause issues for static reference. I believe that you could simulate this behavior by using a certbot deployment hook that does exactly what I've described and modifying your webserver (or whatever) configuration to point to the "1" set in the archive directories.
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