Hi,
The title "Sensei" translates as "one who has gone before", meaning that the Sensei has already been there, done that, gone through what they are guiding the student through, and, hopefully, made the same stupid mistakes before so they can better assist another in the exploration of a system/art/style/way. If the title is bestowed before the person has the requisite experience (in the case of young kids getting high ranking and then teaching without the maturity necessary), then I would personally be hesitant to use the term.
However, I do use the term for teachers other than my own (direct or indirect) as a sign of respect to their skills, knowledge, experience, and dedication. So you will see me use it even for (Japanese) arts I don't train in.