Not really sure where a highly mobile infantry is going to be shooting en masses at 800 yards?
I'm sure the Russians thought the same thing until they were fighting in the mountains of Afghanistan and getting killed from beyond the reach of their issued carbines.
A couple issues are at play that I think are being ignored.
First, most of the planet remains wide open unpopulated areas. Cities are just dots on the surface of the earth so bottom line there is just more opportunity for combat in wide open areas and this will not change for hundreds of years.
Second, a rifle is not ideal but very capable of being used effectively in close quarters situations, however a short range carbine is not effective at distance. The rifle can do both. I'm not saying do away with the M4, just don't do away with the rifle and most importantly the skills needed to employ it. The skill of the long range rifleman has been a key element in the protection of our freedom since the founding of this nation and will likely continue to be for many years to come.
Just my opinion.