Lenses I currently own: 8-16, 17-40, 28-70 f/2.8L, 24-105, 70-200 f/2.8L IS II, 100-400, 500 F/4.5L, 50 f/1.4 (Sigma). FF body.
The 28-70 has always been one of my favorite lenses, coveted in my film days, owned now, fast, and uses the cross focus points to the greatest advantage. It is mildly soft at f/2.8, but sharpens up quickly at 3.2.
The 24-105 came with my 5DC and has been one of my favorites. It's sharp at f/4 and throughout the range, it makes an excellent walk around lens, and when paired with my 100-400, a great grab and go kit with excellent range.
Thoughts: Sell the 24-105 and 100-400, buy Sigma 120-300 f/2.8 OS. Will I miss the excellent range of the 24-105? Will I miss my grab and go full range kit? Advantages? Disadvantages? Any other thoughts?