Streaming of teachers? This is mostly in reply to John's original post. In many fields, not just ringing, there is if anything a negative correlation between being very good at something and being able to teach it. This is of course a gross generalisation and there are lots of exceptions, but a key aspect of teaching is being able to perceive the level at which the student needs help, and then talk, demonstrate or explain at their level, not at the teacher's level. Many experts are not good at this!
In the specific case of ringing, I think it helps to distinguish between teaching basic handling (which, although faults do tend to recur after the initial training, is essentially a once-only process) and then the teaching of methods of increasing complexity. Basic handling is mostly taught one-on-one; there is no reason why it should be an expert ringer or the tower captain who does the teaching; it will ideally be done by someone who enjoys teaching, in which case they will not become bored or frustrated or feel restricted by this task (though they may get overloaded). So I suggest that when John talks about "advanced ringers burned out by doing seemingly endless handling, CC and PH training" they are either being overloaded or they are not very good teachers. In my school-teaching days I was often asked if it was not boring to teach the same maths topics year after year, to which the answer is a definite NO - it is different children each year, each class is different, needs different materials and challenges, and one absolutely does not trot out the same resources year after year. You are teaching people, who are endlessly varied, much more than the topic.
When it comes to methods (and ok yes there is a middle ground of rounds, call changes, plain hunt etc so I am over-simplifying), surely it is more the tower or the group that does the teaching. The student will attend other towers, group practices etc, and while they may on occasion ask their original teacher for comment or assistance, the role of that teacher is very different at these more advanced stages.
So I totally agree with John's comment "I suspect trying to <get more teachers> by drawing basic teachers from the top 10% of ringers is doomed to fail". Indeed, more often than not they would be the last people you would ask!