Let's make an approximation that about 45/50-60% of the RP system have at least one element mastered. Not because one masters an element generally means he is fit to teach it. Your suggestion is like saying everyone with an element mastered should be a senpai. Some members are lucky, they get training in the first month they've joined, and mastered the element that same month as well. That member would be nothing but an inexperienced member with element mastery status, and thus would not be fit to teach. Regardless what rank he can go up to, why send him out to teach others?
The whole problem is that you newer members can't wait. I've joined around this time last year, and yet I don't have everything mastered. Some have joined late last year in October etc. and already have the basic five. Stop comparing to older members, NB didn't have such a large population back in the day, there were less members who RP, less members interested. Yet, you still have members from back in 2009, 2010 who still haven't mastered the basic 5, if yes, then maybe not ninjutsu, taijutsu and genjutsu.
It's the same cycle everyday, everyone gets up and wants to be a senpai. The day they become one, their entire style changes because they simply can't handle the pressure. Not all sensei or senpai are active in teaching, so what? I've been a senpai for about 4 months now, the first day with the title was all fun, accepting students and feeling like I have made an accomplishment. Two weeks later, wallies of training requests in vm, pm, email and msn. Deal with it, I teach who I can. Taking the minimum of 20 students, and at one point I took a maximum of 42 at one time. Everyone has RL issues, some has a heavier burden than others. It isn't easy logging into NB each weekday, knowing a limit of 5 hours until you need to study, etc. That 5 hours I get, I have to teach. I'm not legally bound to teach, I'm not held by a contract, I'm not being payed, but still, I log on, I teach, I do what I do. And yet, there's no difference in results "Train me plz plz plz plz. Please Drizzy senpai plz." Everyday, approximately 10-20 training requests are sent to my profile. How do I deal with it? "Sorry, currently full", if they're generous enough they understand, if not, they continue, "plz plz plz plz". Things and time change, I'm not online during weekdays any at all or only for an hour now, I make up for it my training extra on weekdays. Using an example, on Friday of last week, I decided I'm going to relax, watch One Piece, the end result, "Senpai, reply to training plz plz plz". I'm tired of the "plz", I'm tired of the "train me". Yet, I endure it, I accept who I can, and tell whoever I'm unable to teach that I'm full.
There are too much impatient people these days. I'm talking on behalf of all sensei and senpai here. It simply annoys me when there are silly suggestions for more sensei/senpai as if we're not doing what we can.