@ pull what
Personally I do not think you can be a rounded senior instructor without a background in airline or RAF flying and that's nothing to do with the size of the aircraft, its to do with the professionalism and discipline and the constant goal of finding better and safer ways of doing things. Multi crew flying from both seats also gives you a much wider experience of managing people, which is a valuable skill in instruction.
Insinuating that you cannot be a "rounded senior instructor" (whatever that is) without a background in airline or RAF flying is simply not true. I know many civilian instructors from various backgrounds that are very professional and constantly looking at ways to improve themself as instructor. End of the its down to the individual and how that instructor is managed.