do players improve anymore or do they just work (in a system, or for a manager) or not

Dembele was 24 or so before being recast as a central midfielder and improved in the role throughout his 20s. The entire system was built around his physical attributes though and Poch never managed to adequately replace him.

Recasting big, fast forwards as central midfielders appears to work pretty well in general, give or take the odd Alan Smith.

aw poor alan smith, always felt he was undervalued in any of his many roles tbh

It was a result of Fergie only managing to buy one competent central midfielder in the last decade of his tenure.

rooney was a rb fyi

tanganga was an epl level defender in any position at the moment of conception i dunno if this is relevant