I believe the intention of the OP was that the focus would be on the bottom of the ball which would make sense if viewed from inside the court. These images are focusing on the top of the ball and from outside the court looking in, which makes them less useful. A large percentage of the time players are in the court looking out, not out of the court looking in.

If all players adhered to 6.D.7 and did not call a ball out unless they saw a space between the ball and the line there would be fewer in balls called out. That is obvious. What is also obvious is that in certain situations it requires players to call an obviously "out" ball in, that it increases the disparity in line calling and that this rule is unenforceable.

If the viewing angle is such that you can look under the ball to look for a gap this rule provides good guidance on what constitutes doubt. No gap seen you have doubt. If the player has to look over the ball to see a gap the ball may be several inches out of bounds a no gap will be observed. The shorter you are the greater the distance between the player and ballt the more distance there needs to be between the ball and the line to see a gap. A short player who sets up 2-3 feet behind the baseline to return a hard/flat/deep server may not see a gap unless the ball is 5-6 inches past the baseline. If the receiver (not the receiver's partner) in this example makes an out call when the ball is served 2 inches past the baseline that player has violated rule 6.D.7. It is physically impossible for the player to have seen a gap between the ball and the line either looking under the ball or over the ball. If the opponent appeals to the referee the referee will have to call the ball out, which is OK since the ball is in fact out. Did the team commit a fault because they violated 6.D.7.? If it isn't a fault then what is the penalty? Verbal/technical warning or technical foul for violating 6.D.7.? If there is no penalty, what purpose is there for stating it as a rule? Currently you see instances where a player drives the ball deep through the middle and the opponent clearly does not get their head around in time to see the ball land but calls in out anyway because "it was obviously going out". Do you really think that those teams now will make sure they see a gap between the ball and the line before they call it out?

Players calling "In" balls "Out" is very frustrating and clearly an issue. When 6.D.7 was moved from an etiquette discussion to a rule (2021?) I don't believe it made line calling more accurate, fair or enforceable. It also commonly set up situation requiring a player to call balls that are clearly "out" as "in" if no gap is seen. 6.D.7 should be moved back to a discussion of rule calling etiquette and guidance should be developed for situations where the player is looking over the ball rather than under the ball. Examples would be for the receiving the serve, sideline calls when the ball lands between the player and the line and examples when the player is looking down on the ball.