Allow me to shatter this conversation into splinters.
First off, I believe account expiration is a terrible idea because I have seen people on forums who have left for 4 years and come back before. Account expiry is something that happens with clans moreso than forums. For instance, I was once apart of a pretty pro-tier clan on Guns of Icarus Online. After one month of not logging into either GoIO or their website, my account was deactivated for more than a month of inactivity without prior notice for my absence.
Now, there are websites that I haven't touched in years (literally, there are some forum boards that I haven't touched in forever). But when I decide to go back to them, is my account deactivated? No. More often than not the site has been removed from their host for an extended period of inactivity (or because the host decided to shut down the site/no longer pay for the service/change their domain name). There are times though where I've logged back in, only to find the community still booming without me (Minecraft Forums for example).
I believe that the thought of account expiration is really unjust. Simply because that sort of feature being added usually means the community you are apart of is very high-profile, or because the owner is begging for advertisement views (more ads = more $$$ for the owner to pay the hosting service). Even then, some forum communities prefer to keep their members there, simply for the fact they can say they have/are apart of a forum community with over 2,200 members.
Long story short, it's just a terrible idea. People can leave and come back after many years on a forum, and the implementation of it is really unjust. Plus, we all like to see big numbers of stuff.