It's been a while since I've been actively participating (if you could call it that) in the MediaWiki project, but judging from a quick glance at the trunk of WikiEditor, it's still actively developed. Not as actively as VisualEditor, but it's still getting bug fixes and being kept up-to-date with MediaWiki releases. WikiEditor is also pretty much "finished", so I'm not sure what else would be added to it other than maintenance fixes.
Don't treat this as an official statement or anything other than speculation, but I'm pretty sure WikiEditor is going to be around for a long time. VisualEditor is still "unstable" and some people like myself prefer WikiEditor by far (I've actually disabled VisualEditor on every Wikimedia site I'm a part of). Additionally, VisualEditor relies on Parsoid which in turn relies on Node.js, and I'm pretty sure WikiEditor would be kept around so that MediaWiki installs won't require both PHP and Node.
However if you're a new dev wanting to see how MediaWiki works, definitely ignore VisualEditor for now. You want to learn about MediaWiki core first, and when you get to extensions you'll want to look at simpler extensions. VisualEditor has a ton of moving parts that aren't necessarily tied to MediaWiki, so it's not going to give you much information that other extensions can't.
As far as what the main goals of the MediaWiki developers are, I don't really have a clue. I sort of just started fixing what I came across that was broken, or adding things that I wanted to a handful of extensions. I never really did figure out where all the developers hang out, except maybe IRC.