Well, to start off, we do sometimes turn PCs into NPCs, both the development and the DM team have to vote on it, and the key criteria is how does it enrich and contribute to the world. If a DM wants to use a closured PC (with the player's permission), then they put it to a vote; it's generally easier to become a DM-controlled NPC than being added to the module.
If we convert a PC into a module NPC, or retain some legacy, as mentioned, it has to enrich the world, add something that's missing. A few years ago I was in talk with a player (before we added the class) that wanted their Voodan PC to open a Voodan shop in Dementlieu, a very cool concept. We could have turned the PC into a NPC as a fixture since it would have added something; however the player didn't want to let go of the character, so it fizzled.
And we do have a few memorials, and places where we can and do list the names of dead PCs. The Ezrite Mausoleum in Vallaki is such an example, that we update from time to time when Ezrites kill a memorable villain/MPC. However, over time, just like any other grave markers, the names become meaningless, just random names one passes by. After all, who actually remembers or is affected by Adeon Pappas? Who remembers the plots the PC was involved in? And in the end those who do remember the PC don't need a memorial or a grave marker.
Finally, while I don't want to go into detail about this specific case, I'd be very uncomfortable "rewarding" a MPC who, rather than closure at the hands of adventurers, stepped out of the role, which is the very opposite of closure. Doing so would encourage MPCs to hold onto their character, whereas we want them to take risks and enrich the world. The 6 months timer was added so they wouldn't want to live forever.