If you look at the evolution of the play-the-ball, there is an argument to support cutting the numbers down from 13-aside.
From 1906 when the play-the-ball was introduced to c.1950, the play-the-ball was truly a contest (even more than the ruck in modern RU today). As a result it kept all 12 forwards within very close proximity to the ruck/play-the-ball. The space available to the backs at EVERY play-the-ball was roughly akin to what we see today at a scrum.
From 1950 onwards, players were increasingly getting away with cheating at the play-the-ball to keep possession - while there occasional instances of the ball being raked back by the marker (even into the early 1990s), for the most part the team in possession kept the ball. Proof of this was the bash-and-barge era of the early 1960s - which could only have existed if the play-the-ball was hardly a contest.
The point I'm getting to is...when the play-the-ball was a real contest, attracting all 12 forwards, there was space everywhere for the attacking team.
During the 1950s and early 1960s, with teams knowing that they were less likely to win the play-the-ball, only 2 forwards were committed to the ruck area (marker and 2nd marker), with the other 4 forwards then taking up places AND SPACE in the defence line.
This led to the need to set a minimum distance for the defence to stand back, and has now evolved to 10m.
If you watch RU today, you can see the same trend developing - the ruck is for the most part not really a contest (despite what many RU people say), and defensive forwards are not all getting involved at each "break-down", but opting to stand in the defence line. For RU, having 8 forwards instead of RL's 6, means that attacking rugby is even less likely - hence they have been forced to look at their rules (the ELVs).
If RL teams were dropped to 11-aside, then the 10m would definitely have to be cut down to 5m.
American football came from rugby (mid-1870s), and almost from the outset made the scrimmage irrelevant as a contest for the ball (as we have now done with the scrum and play-the-ball), and changed from 15 aside to 11.
The risk in cutting down RL teams to 11, is that very soon after RU would drop to 13 - they don't do it now as RL and 13 aside are synonymous with each other.
The advantage of RL teams dropping to 11 (with a 5m rule) is that expansive ball-passing play is more likely. Additionally, as there was in 1906 when RL dropped from 15 to 13, there is an enormous economical benefit to clubs - it is cheaper to field teams with 2 less players. It would also be easier for RL at the semi-pro and social level to play, and to fill teams.
I don't think dropping to 11 is a ridiculous idea, but it would be a seismic-shift in the code's history, and a step not be taken without a lot of trials and consideration. The influence of the $ should never be ruled out in a pro sport.
12-aside RL was played in NU competitions in the 1903/04 season:
http://www.RL1908.com/articles/1904.htm
