I have owned and used the original 16-35 f/2.8 ver. 1 for years. For all kinds of indoor, low light and outdoor stuff, campfires, etc. But even f/2.8 is too slow for campfires. I got a EF 28mm f/1.8 prime for campfires. Lately I've been trying to get used to a better 35 f/2 IS prime to replace it. I also like wide lenses so it's tough.
The point I'm making is that I LOVE the 16-35 f/2.8L ver. 1 and since I got the 16-35 f/4L IS a year ago, I rarely use the f/2.8. Because when I use the f/4L, I can bump the ISO a bit in camera and bump the exposure a bit in post and I'm done. The f/4 is a better lens and renders a better image.
If I'm in very low light, f/2.8 barely cuts it anyway. I reach for a faster prime lens and start working harder with keeping still, using manual focus and taking a LOT of shots. You can also maybe cool off a diffused/gelled flash 2-3 stops and get some interesting campfire shots.
BTW, don't go nuts on finding a fast L prime chasing better low light shots. I owned a 35 f/1.4 and 24 f/1.4 II while I was seeking the 'ultimate' campfire and dark environment lens and honestly, shooting things in darkness pretty much negates any advantage you get from a fast $1500+ prime lens. And once you get past f/1.8 the thinner DOF makes it difficult to get much in focus anyway. And focusing in the dark, often manually, can be a bitch. Just get the fastest EF you can afford in the focal length that works best for you at the campfires and call it done. Then use the great 16-35 f/4 IS for everything else... indoor, etc.
Have fun and let us know what you decide. Share a few pics! Yay!