Aye, PbP tends to struggle a bit in this regard - in my 12+ years of playing like this, I have only been in 3 games that have managed to end the story successfully, others simply kind of vanished due to inactivity/some other problem. Been in 105 different games on the previous server, so the chances of having a game come to conclusion are less than slim.
I actually GMed one of the finished ones and out of the original players (8 or so if I remember correctly), only 2 managed to stay with us until the end, with one going on a few months hiatus (the game lasted about two and a half years). And I am not even counting those that have come and gone, that would be 15-20ish players.
Also happy this game is still going! We'll see how we manage to finish the Sepulcher, I was actually considering bringing on a few new players for the next adventure, running a new funnel parallel to your adventures and giving those with few surviving characters an opportunity to get some new ones!