Depay came here looking like a world beater, we paid a fee that reflected his position as one of the top young players in the world, now look at him, would this decline have happened if he'd joined another top club? Maybe, but I wouldn't mind betting he'll improve drastically elsewhere, ADM was fantastic before he came here, ok he's not been as good for PSG as he was or Madrid, but he's a darn sight better than he ever was for us, Schnierderin came here as one of the best DM in the prem, Darmian was thought of as real coup when we got him, now they look average at best.
Whether we are buying the wrong players wasn't my original point, it was more that we don't seem to have been able to improve any player we have bought post Fergie (Bailly aside), and alot have regressed, it's become the norm.