Any tees stretching over 6800 yards should be played with permission only. 6800 is plenty for anyone, even those 15 handicappers who can hit it 280+.
If a guy hits a driver 300, hits with some consistency, and is a 15 because his short game stinks, is he really going to (a) have more fun or (b) play appreciably faster if he plays a course from 6700 yard tees instead of 7000 yard tees?
This shouldn't be about "permission." It should be about convincing people that there is no honor in slogging through a golf course at a length that isn't appropriate or manageable for their games, and giving them the options they need to play the course that's best for them. That depends on ensuring that forward sets of tees are well-planned and give the player a comparably fulfilling experience to that of a longer hitter playing from the back tees (if not necessarily the same experience on each hole, a goal that just doesn't seem possible for the majority of holes).
That's not to say that it wouldn't be good for the game if more courses topped out at 6000-6500 yards. One of the biggest problem for a long hitter -- say, my hypothetical 15 'capper -- playing middle tees is that he very rarely gets to play a very long hole, because the longest par 3s, par 4s, and par 5s tend to be the holes that have the greatest differences between tee positions. Anecdotally, it seems like shorter courses have a much greater variety of hole lengths than the middle tees of longer courses, where the total length is the same.