Why does a course's back tee length matter so? If people would simply play the tees that suit them, then anything 'behind' them wouldn't matter. I used to play tips, but now find that my age is better suited to the next shorter tees. My rule for the correct tees, for myself and anyone else...what tees will provide the greatest opportunity to use all or most of the clubs in your bag. That rule would have me tipping most any course at 6800 or less, but maybe using the middle tees at some of the newer, lengthier courses. And total distance still is not the greatest criteria, as conditions might play short (wet/cold/hilly) or fast (dry/altitude).
As evidence: Merion. New tees for the Open will have NO effect on members who will still play their usual tees. Moreover, a vital element of the game's growth is young/new player development and they want to hit the bombs, so let them stretch it out and enjoy. What is far more damning to the 'average' player are designs or renos that create forced carries, brutal rough and deepening bunkers.