You probably have a point. But fitness has nothing to do with taking dreadful set pieces and giving the ball away so cheaply. These are basics for pro footballers who also btw, have a very good base level of fitness. Your point would excuse a lacklustre end to the season or even end of matches. But to excuse such consistent, dismal form across most of the team on fitness is unrealistic. Considering sidibe didn’t have any pre season and plays with more intensity than most would also put that argument to bed.
Also, did you see the Chelsea and Man U games? The fitness and intensity is there....