Allow me to disagree on this one, Steve.
First, short observation about Launius: agree, I don't know either if he ever resulted an official employee, but I can guarantee he's still around designing /helping designing games (I worked with him during one beta and one alpha in the last year).
Then Kevin. It is true that Kevin left, but he was responsible for several product lines, and some of these were continued by others: Wiz-War received two expansions, one designed by Jason Little, and another one designed by Nathan Hajek; Cosmic Encounters' new expansion was recently announced (fan expansions having an official release is something that could be interesting for Arkham's future, I think). This clearly wasn't the path followed by Descent, but Descent had many problems so that a new edition could be justified by this reason. Elder Sign got an expansion too, designed by Kevin even if at that time he was already away from FFG, and both Christian and Kevin said that the relationship is good and they could work together occasionally in the future.
So, I'd not link Kevin abscence to the destiny of Arkham. Arkham is just a very complex game that was expanded in the years to its limits (an additional board is improbable at this time, and so new mechanics); new content could arrive in form of Heralds & Monsters packs, or thematic mini-expansion, just like Forbidden Lore was for Eldritch.
Regarding Eldritch... I don't see it as a spin-off due to the imposibility of expanding Arkham, but more as an answer to those who asked to expand Arkham's life beyond Arkham. Several times fan asked about going to Antarctica or the Pyramids, and having a worldwide perspective of the Mythos, and Eldritch is great under this point of view.