As I get ready to buy the 6D with the 24-105 lens I'm wondering why none of the other manufacturers are offering a mid range quality zoom, maybe something like a high quality 18-135. Canon seems to have had this segment to themselves for a long time and I'm a little worried the 24-105 is "outdated". Am I justified in thinking that and is there anything on the horizon from other manufacturers?
It's still a great lens to "carry around" if you don't know what you might encounter. A bit more barrel distortion on the wide end than I like, but easily fixed in post. It's a great bargain in the kit for an extra $500.
As for making an 18-135 lens, and making it well, that would be expensive. The zoom ratio is that much higher.
Before Canon updates the 24-105 they may update one of the other lenses in their "f/4 L" series. The 17-40 is a few years older, and the 300 IS is really old (though still a good performer, it could use updated IS at the very least).