Has he now.
I think the truth is, Harry was always good at identifying what he needed immediately in the short-term. If that is the metric, then OK. But I would not call the likes of Saha and Pienaar especially astute buys, just expensive but 'camera-ready' squaddies.
Postecoglu started his first season with Vicario, Romero/VdV and Bissouma as his spine. He was shorted a CF at that time. He most certainly did have structure, and a foundation/principle. You just don't like it. Or his style. Or him. Whether his style literally has legs is certainly an important discussion. I personally think he has lost too much in terms of energy and momentum via the disruptions to this season to last; I think he's gone. But to question his 'interest' in fundamental principles is just silliness.
I get really really tired of this 'amateur hour' nonsense. When will you and others accept that you do not have the career/experience he has by being an 'amateur'? You don't like his style/way we set-up? Wonderful, have at it. It's the extra jabs which (for me) weaken that opinion every time they get thrown. Frankly, it smacks of ignorance.