My interpretation of your statistical analysis: Partey is someone who is physically strong, can perform the defensive duties of a CM while being dynamic enough to sufficiently progress the ball forward.
Yes he’s probably not the best at anything, but he is very good at everything. Put him around two midfielders who complement his style and he can be a real powerhouse in the engine room.
Is he worth 50 million? Yes, because any team will be stronger with a player of his all round ability.