Alonso still has that required hunger and dedication, something I think Yaya (if he ever really had it to the same extent as Alonso) lost it a couple of seasons ago. His ego dented his quality.
If his mindset is on track, the diminished physical aspects won't be too much of an issue.
And Alonso does a shitload of hard, dirty and undesirable defensive work, supporting his center backs AND fellow midfielders.
Does Yaya has that in him?
I think not.
I very much doubt Yaya will ever again play 90 minutes of high intensity football, and I suspect that is what Pep will demand from each and every player.
I personally hope the big man moves on, allows for us to introduce a younger player in his place, AND I have this feeling it would benefit Nasri if Yaya left. Just a hunch.