Fer has been important to us in the roles he's been asked to play but he's never struck me as being 'fantastic' for us. I get your point though Roof, the boys done good for us. Good in some games, ok in most, but here's the rub, he's been non existent in a few too.
He's not consistent enough to take Siggys place at AM and when he did he was wanting imo. The success of most teams to a large extent depends on their AM and how effective he is. An AM is like a #10 in rugby or a QB in the CFL where a team attacks through them. A good AMs pulls the strings in attack and directs the direction and tempo of the game. Fer may be adequate as an AM but his lack of consistency marks him as 'ok' in that position and little more, imo. Fer fades to often for my liking to take Siggy's spot but, if Siggy did leave us then the job is his I imagine, simply because he can be good there, he can pick a pass but he also loses focus and gives the ball away too easily.
If I had to chose between Fer and Siggy at AM then I'd chose the reliable, inspirational, workhorse that is Siggy every game.