Doubtful. As cameras get better at doing high-ISO shooting, the need for fast lenses is decreasing, not increasing. It would be utterly absurd to stop selling the 24-105 as a kit lens in favor of lens with only 2/3rds the zoom range just to go from f/4 to f/2.8.
More light isn't the only reason for a fast lens. The thinner DoF with f/2.8 makes such a lens useful for portraits, and a zoom lens more suitable for portraits is very nice to have. Not that you can't get decent subject isloation with f/4, but you need a closer subject (maybe not the framing you want) and/or a more distant background. I find the 24-70/2.8 and 70-200/2.8 more useful than their f/4 counterparts mainly for the better subject isolation.
I tend to think of portrait shooting in terms of primes with much wider apertures (like f/1.8 and down), but that's certainly a fair point if that's a lot of what you use a zoom lens for.
...Canon doesn't currently build anything longer other than the excessively large 28-300L.
I'd really like to see the next kit lens be either an IS version of the 28-200 or possibly something slightly wider—say a 24-200L f/3.5–f/5.6. That would be the perfect walking-around kit lens—long enough and wide enough to get all but the most extremely long shots without having to change lenses.
The 28-300L is essentially the same size as the 70-200/2.8 or 100-400 - it's not a small lens, but I wouldn't call it excessively large.
The problem, at least for me, is that it is just so long even at its shortest length. Its minimum length is almost double the length of the 24–105. I carry around a 70–300L, and I find that to be annoyingly long and hard to deal with. The 24–300 is as long as the 70–300L with a 2x teleconverter attached, give or take.
And it's heavy. At 1.47 pounds, I can walk around with the 24–105 all day and it isn't too bad. The extra 0.84 pounds of the 70–300L is kind of clumsy. The 24–300 adds another 1.36 pounds on top of that—just shy of my 24–105 and my 70–300L put together
. That's just not a walking around lens. By the end of the day, you'll feel that hanging around your neck.
From what I've seen, full-frame lenses with a given field of view usually seem to weigh anywhere from 1.1x to 1.2x the weight of an EF-S lens with the same field of view, though at the ultra-wide end, the 16–35 L II is about 1.6x the weight of the 10–22. So given that an EF-S 18–135 weighs about a pound, I'd expect a 24–200L lens to weigh somewhere between 1.1 and 1.6 pounds—less than my 70–300L, and not significantly more than the 24–105. I wouldn't expect it to be as long as the 70-300L, either, but I could be wrong.
In other words, I think that range would make an amazing "grab it and forget it" lens, particularly if equipped with a decent IS system.