If the “speed of the opponent being screened” is a factor does that mean bigger, slower defenders are allowed less space than smaller, faster ones? If the legal screening position “will vary” then doesn’t that mean these definitions are arbitrary and really just come down to the judgment of the official?
Now, when a coach doesn’t like an illegal screen called against his player, he can call timeout and challenge it. This will send officials scurrying to the monitor to look at a replay. Will a “normal step” and “speed of the opponent” look different in slow motion? If the definition of a legal screening position varies, anyway, then what’s the point of checking it twice?
Proponents for replay reviews in sports usually say it’s worth it to stop games to “get it right.” That’s a reasonable view when “right” is clear and easily assessed. It can be easy to see if the running back fumbled before his knee touched the ground, who was the last guy to touch the basketball before it went out of bounds and whether the runner reached the bag before the ball was in the first baseman’s glove.
But some calls come down to referee’s individual interpretation of rules that no one has ever been able to clearly define. Often that’s the case for pass interference in football and personal fouls in basketball. The NFL and NBA can have their coaches challenge calls and send their officials to monitors as often as they like but, at some point, judgement calls must be allowed to stand so we can get on with the games.