The Hawks have won 3 Stanley Cups during his tenure. I think he's already proven that he's a top GM.
There's different levels and assessments to this. He had available to him the core that was already in place, and the money to strengthen it. Stan joined after the core was in place. Credit where credit is due, his early signings helped the team transition from a young talent filled team, to a perinatal contender (and favorite), but the true mastermind behind the backbone of the team was Tallon.
It was not sustainable, and the aftereffects started to take their toll about 3 years ago. Bowman benefited directly from his predecessor.
I think it's become increasing clear that Bowman isn't the mastermind everyone thought he was early in the dynasty. This year is going to prove whether or not he actually is a good GM. I'm inclined to think he is not. Contracts, trades, and drafting have all been highly sub par.
An even more interesting question is whether or not Q is or was an elite coach. I for one, credit him for a lot of what the hawks accomplished, ten fold to what Bowman did. Now, he also might be the reason they fall into an abyss, but im more comfortable with giving him more credit and time to turn it around than Bowman. Sadly, the way pro sports teams typically work, he will be used as a scapegoat, at least initially.
Honestly, the real reason for almost any dynasty is mainly luck.