I think you need to look at players from a team perspective. Football is a team game. Your not going to play a full midfield of people who get assists and score all the time. For every Geovanni your gonna need an Ashbee in there too. In 2011/12 McKenna played in the midfield mainly with Corry Evans and Koren so it wasn't his job to make that difference attacking wise.
Not all players have to do something individually amazing or standout to make them effective and to help win a football match. Working hard is part of winning the overall battle. Not all players are in the team to specifically score and get assists.
Quinn made his impact in games by helping us keep possession and by hurrying the opposition.
That's exactly what other teams have. Diame aside, how many of our midfielders scored goals last season? It was a major failing in the side and everybody pinned the goal ratio on our strikers, but most other teams had goals coming from other places. I agree you need a 'midfield general', but you need one that influences a game and Quinn just didn't do that enough for me