Neither Barkley nor Sigurdsson can be relied upon to carry out the grunt work needed from a CM. Gana, Davies and Schneiderlin however all have the potential to add goals to their game and when do we ever set up with 3 DMs anyway? Look at the game currently being played: Barry is sitting as a DM but Gana and Davies have both been deployed as box-to-box CM types.But you don't need 3 of them as they don't score and don't create hence why you need a CM what can take the ball and link play, but also get goals. You'll struggle to find a team in the top 4 that has more than 2 DMs.
And Sigurdsson as a CM he'd work well in that at as he's very adaptable and can interchange with forward players like Barkley. Its about fluidity with the midfield and attack that we don't have.