See, now, I think it's pretty clear that 1,700 yards is beyond the limits of an ethical shot.
Why? Because there's not a gun in the world that can group smaller than an elk's vital zone at 1,700.
To me, that's the limit. ToF isn't the factor. For one, as per the OP (if we take that scenario at face value) even 100 is too far in terms of the animal having time to move. Two, at long range, we have no idea if the animal moving will be bad or good. One step makes it a bad hit. More steps makes it a clean miss. Clean miss is good. Might be better off at 500 than 100 in some situation that is completely random and unpredictable. Three, if ToF is the issue, almost the entire history of bow hunting has been unethical at almost any distance.
The limit should be that range at which the hunter can no longer guarantee within an acceptable % that the bullet will land in an area within the animals vitals as they are when the shot decision is made.
In almost all cases, that puts the limit well below 1,000 yards but it's not at all hard to imagine pretty easily above 500, gun and shooter dependent. Very few guns can guarantee a bullet inside a game animals vitals at 1,000 yards regardless of the shooters skill, a great many can do it at 500.