Some months ago I asked advice on what to take to Borneo on a bird safari: the 400mm DO II or the Sigma 150-600mm C for my 5DIV (the wife has the 100-400 II + 5DSR). There was no response but I made the difficult choice of taking the prime, and buying an ultralight case for hand luggage. It was the right decision as I needed every stop of light and the best IS as I was shooting in the near dark so often - early morning, dusk, late at night with a spotlight, and in a rain forest. There are those who are still sneering at IS. Well here are some efforts at f/4, with iso6400 on the 5DIV and speeds down to 1/20s hand held in a boat. Try that with a 400mm f/5.6!
The short-tailed greenmagpie, whistling thrush and little spiderhunter were early morning, and the buffy fishowl and blue-eared kingfisher on a night trip in a small boat with a handheld spotlight.