Years ago I have used the 70-300 IS but it was replaced with the 70-200 f/4 IS shortly after. The 70-300 did OK but the 70-200 with an extender was much better. I no longer have the f/4L.

Currently, I use a 70-200 f/2.8 II with a 2x III extender. So it really becomes a 140-400 f/5.6 technically which is great for spotting (much like a 100-400 would be). I have had great results with this combo on my 5D3. I have used other telephotos in the past though, such as the 300mm f/4L IS and the 400mm f/5.6L - Both excellent lenses... But I find zooms a bit more useful when shooting aircraft. The 70-200 + 2x has been much better than expected image quality wise.

But in your case, if you sell the 100-400 for the going rate of what, $1200 or so? That leaves you very little for any telephoto if you purchase a 24-105 or 24-70 (which would cost $900 to $1000 or so)

I would honestly recommend keeping the 100-400 and just replacing the 17-85 with the 18-55 IS if you needed to unload the 17-85... Costs what, $100?  And IQ is every bit as good as the 17-85, which I have owned in the past. The aperture ribbon failed inside of it on mine. I believe the AF failure is common as well with a similiar ribbon issue. Why not send to Canon and have them repair it? It may cost $100 or so, but the lens will be like new all over again if you like it. Canon repair is excellent, especially the NJ location.