Never said Sami is terrible or mediocre, he's just lazy and that affects his game big time. I don't believe his decline last season and this one compared to his 2nd half form of 2016/17 is due to physical/tactical reasons, he's just not giving his 100% in most of the games. He still has the very good games from time to time, but they're too few and too far inbetween to be a starter
Look at his games in Serie this season f.e., the only one where he lasted the full 90 was against Sassuolo, and dude got 25(!) successful passes in 90 minutes. That's just hiding from the ball and you know it, if a midfielder refuses to get involved in the buildup, I don't know how else to describe him apart from lazy. It's not like he has less quality on the ball than Bentancur f.e. (65 passes in 90 minutes against Bologna), but "pure class" Sami would rather just let others do the work and pop up when the ball is in final third.
I wonder what others think about him. Clearly he is a very good midfielder who has a lot of qualities and has played for some very good teams and won a lot. Yet, even more clearly, he currently is not up to standards to be a starter at Juve, even his most vocal supporters say it. If it's not because of his effort lacking and if it's not because of lack of quality, then what is it, being 31? Surely that's not the age to give up on football if you're a pure class hard worker like Sami.