The idea is to send a message that account farming on this scale (or any scale really) will no longer be a profitable endeavor.
What's 2 years to an account farm that hasn't been used in 5.5 months?
Are you
sure its not being used?
Its almost impossible to find a reputable source on Google about "passive SEO link saturation", but if you put that into duckduckgo or whatever you use you can probably glean what its all about.
Such links have no special value in the long term. To have a positive impact on SEO, the page (profile page) where the link is posted would have to have some kind of views, and these are looks like archived.
But they can certainly help with new domains when search engines do the first indexing.