Do numbers define the code?
Arguably, RL teams are 17 now, not 13.
Maybe 11-a-side with 2 fresh inter-change reserves is still truly a 13-a-side game?
The idea of allowing any replacements at all beyond the starting 13 didn't become the norm until the 1970s.
Interestingly, in 1903/04 the NU were playing 12-a-side for the County Championships and most junior and business competitions. The first international match was also 12-a-side:
http://www.RL1908.com/articles/1904.htm
Let's not also forget the reasons teams went from 15 to 13 (and before that 12):
1. To provide a better spectacle than RU, and to compete with soccer for spectators and players.
2. It was cheaper for clubs to operate, and easier for lower clubs and juniors to form teams.
RL doesn't live in isolation like AFL or American football - money and the context within which RL competes (against other codes) means we have to look at all options.
I agree, after 102 years going away from 13 would have to be a vary carefully considered step, but I wouldn't rule it out.
And it's not just because players today are bigger/faster that there is a case to reduce numbers - the play-the-ball "pocket" prior to WW2 attracted EVERY forward at EVERY play-the-ball - they weren't bound as tightly as in a scrum, but when the play-the-ball became less of a contest (from 1950 onwards), forwards began spreading out across the field, taking up space that backs once had to themselves.
I'll be reserving judgement after the trials (though if they keep a 10m rule I doubt its success).
More comments here:
http://www.RL1908.com/blog/11-aside-rugby.htm