Shooter's skill + equipment = ethical range
To me it is that simple. There is no one size fits all. I just watched a military show where they were using a new .50 caliber rifle and made kills at 2400 meters. It would be foolish to impose an "ethical" range of 400 yards on that solider provided he was deer hunting with a rifle with which he had practiced and was capable of long range shots.
The problem lies in being true to one's self; knowing your limitations.
Until this fall, I had never hunted with a rifle and had not shot a rifle for over 20 years. I put in for an antelope hunt in March, bought a rifle in April and spent from April until October practicing every chance I could shooting at 200+ yards. When the time came and that goat was 300 yards I felt confident that I could make the shot. I knew my rifle, the ballistics and had practiced, practiced, practiced. I put the bullet exactly where I wanted and dropped that goat.
I think my 300 yard shot was very ethical.
However, there are "hunters" who do not practice, go out only during deer season and try to shoot a deer at long range. A co-worker took 4 shoots to kill his buck across a field. 2 misses, 1 wound and then finally stalked up close enough to kill the wounded deer.
We can talk here all we want, but we are never going to stop those hunters from taking the long range shots.