It will still happen, but I've updated my priorities so it's been pushed further down the list. This is not a bad thing though, it actually means that the more exciting bits can happen sooner.
The plan WAS that QuestKit would be the next version of Quest. So there would be two systems:
Quest 5.x - a primarily Windows-based system, with a web-based runner and editor (using lots of server-side logic, so they're slower than the offline Windows version).
The new plan still starts and ends at the same place, but it has intermediate steps.
Quest 5.x - as before.
Here's Quest 6.0 on a Mac running The Mansion II, a game written for Quest 3.5 and published in 2006...
Depending on how much spare time I get, I hope to have the remaining ASLX support done in the next few months. That should hopefully mean Quest 6.0 will be released some time in 2016. Then I'll start on Quest 7.0 which will take a while, and then finally I'll get back to QuestKit.
The plan for the future of Quest has changed a few times, as you'll see if you browse older forum posts, but I've actually written some code now so this feels like a much more concrete plan than before. It delivers a lot of the advantages of QuestKit sooner - faster online games, the ability to package a game as an app using the same runner etc. Unfortunately it means QuestKit itself is delayed, but I hope that's an acceptable trade-off!