What I think you are saying about the advantage of a derailleur system is the range of gears and there is some truth to that. The range from high to low is much greater than an IGH. Where it is usually compromised is the gears you need for hills. It certainly something to consider, though there are work arounds to this.

I come from the school of thought of what are you going to be doing with your bike 80% of the time and work around that. I wouldn't buy a derailleur bike because there was one hill on my route when the other 95% time it is flat. I worked in the bike biz for many years and I was always amazed at what people thought they needed vs what they really needed. I spent more time educating people on bikes than the actual sale.

I have been thinking of building my girlfriend a bike with an IGH but worry about the hills for her. Currently she has a triple up front with a bailout gear in the rear (maybe 32) and she uses it where we cycle up and down to the river. What I think I can do is go with the IGH but change the cog on the rear to something larger or change the chain ring up front. This way we lose on the top end, which is fine since she doesn't go that fast and gain on the bottom end, where she will use it. She will never have the same range as her current bike but that is ok, because she never uses the top end of the range. I have never seen her shift to the largest chain ring. You could do something similar but you may not have to.

My vote is still for an IGH.