Forcing NA Duty Finder to be better, we need a grading system.

[Tolmos]

Honestly, this would probably work. It isn't hard-locking anyone; as the OP said, if you don't have people of your own grade to play with, it will adjust to get you into a group. But it will prioritize putting people in your own ranking level. So just because you are an F does not mean you will never play again, but rather that it will try to group you with fellow Fs first, then Ds, then Cs, etc.

This would be useful even for the Fs. One thing that is a big turn off for many MMOers is getting into a group with people outside their play skill / style range. If you are an S ranked player and you are playing with an F ranked player, you will feel frustrated. On the same token, if you are the F ranked player with an S rank trying to rush the group and getting upset at your mistakes, you are going to feel stressed. Neither player in that situation is particularly enjoying themselves.

[Kallera]

Honestly, this would probably work. It isn't hard-locking anyone; as the OP said, if you don't have people of your own grade to play with, it will adjust to get you into a group. But it will prioritize putting people in your own ranking level. So just because you are an F does not mean you will never play again, but rather that it will try to group you with fellow Fs first, then Ds, then Cs, etc.

This would be useful even for the Fs...

The problem is that everyone is then punished for the lack of progress achieved. say you were that s ranked player, but you are looking for a group at the wrong time, so you get saddled with the players doing poorly. if you stay with the group, you will risk losing your standing as their wipes get counted as your own.