Fully agree that in-game coaching is extremely overrated..if you go on any NBA forum, you'll see that literally every fanbase complains that their coach doesn't make enough in-game adjustments
I disagree with you about creating a system, though..I think it does make a difference when somebody like Kerr or Stevens joins a team and creates a system for them to utilize..
However, once the players know the system and buy into it, the coach becomes mostly irrelevant..you could replace Kerr with Lkrfan and it wouldn't matter, since his players already know everything there is to know, at this point..
Talent ultimately wins, though, of course..even a team like the 2014 Spurs was loaded with talent, it just didn't look conventional due to the lack of traditional, volume-shooting scorers..