Perhaps it would be good to apply some sort of penalty to accounts that stop interacting for a long time (years), i.e., lower their rank.
Since sometimes it can be difficult to tell if it's the same person starting to post again or not.
Looking at this suggestion from a different perspective, what if the user was sick all along and couldn't engage in forum activities for years only to recover and resume forum engagement just to find out his/her account got a lower rank don't you think it worsen the state of that person. This is a community and in a community we don't chase or bully people for being absent for long, we only protect against impersonation.
Or a simple warning in their signature, on service, lending, or marketplace boards, indicating that they are interacting with an account that has been dormant for a long period of time.
This might be a better suggestion, a warning badge, logo or message showing on every post they make indicating a user have been absent for a long time just to keep forum users safe from scam.