We had a similar discussion about this campaign some time ago. My face to face experience is limited; the last time I played tabletop was 16 years ago. But I know those games have player attrition, irregular attendence, and missed clues. I don't see that PbP is any worse. In fact, given that in PbP everything is written, it is possible to go back and pick up these clues. Dedicated players can stick with a campaign even if they move to a different city. In some ways, PbP is better.
I hope you are able to continue this game, and that the other players will stick around for the long haul as well. Short, concise games are much easier to pull off, but also less rewarding than a longer, more intricate plot.
Personally, I no longer join single-DM games. They are hard to keep going long enough to take PCs up multiple levels. Individual DMs burn out. (I know - it happened to me.) Persistant worlds give PCs the chance to play under different DMs, and have a much better chance of reaching high levels. DMing in persistant worlds is also easier in some ways. I'm running, or trying to run, a long, meta-plot game in living EN world. I've broken it up, just as you say, into episodes. I'm on #3 now. I try to make them short enough to keep me from burning out, but each deals with the same long term plot and has a number of reoccuring NPCs. It takes a lot of pressure off of me to be able to drop DMing for a spell, and reassess my progress.
I know you had a negative experience with LEW, Silentspace, but there are other persistant worlds out there. My favorite, actually, is
D20 World of Greyhawk.
In short, I hope you do continue this game as it stands. But if you choose not to, I hope you'll continue to DM in PbP, perhaps in a persistant world. They're always looking for good DMs. I'd join any game you DM. Just let me know!