I think the problem would not be that people would not want to play as a defender, I would have no problem for example. No the problem is that those of us who play as defender, and then would watch others play upfront, if we felt that they were not doing a good job upfront, and that we could do better. If they are not good upfront, then you will have problems scoring of course.
Its like in the 5v5 now, if you get idiots playing forward and who cant shoot or dribble, well it pisses me off. So if you had a team with friends only, and you trusted them to be up front, I think that would work better and people would be willing to play as a defender.