I'll play devil's advocate for a minute here.
At 20 yd's and under, an elk is more apt to see you, to smell you, to hear you, to hear the string drop. Other animals that may notice you are more apt to put the elk on alert; all of which could cause unpredictable behavior resulting in a bad shot. Therefore, there's no way any ethical hunter would even consider taking a shot at an elk under 20 yds.
On top of the variables listed for elk, at 20 yds and under a deer may drop, jump, spin, bolt, kick...or just stand there when the string is released. There's no way to accurately predict what they might do. Again, no way any ethical hunter could consider taking a shot at such a short distance.
Of course I don't believe anyone lacks ethics for taking a shot at under 20 yds, but the points are valid if you want to argue them. I actually saw an article years ago where a famous bow hunter made that argument about shooting at deer.
A 50 yds shot can add to the variables, but it can also subtract from them. There will always be variables beyond our control. Depends on which ones you are comfortable with as to whether or not you take the shot.
As I said before, if we were to wait on the absolutely perfect, fool-proof, can't miss shot then we would never take a shot.
Not to say we should take risky shots hoping to get lucky, but as has been noted: some can shoot more accurately at 50 yds than others at 20.
Why is it more ethical for the lousy shot to make an attempt at 20? Why is it more ethical to take a shot at squirrels, rabbits, carp, stingray, coyotes, pigs, etc. vs. elk? Either you are trying to kill it, or you aren't. Right?