The coach having some say on personnel is standard operating procedure in the NBA. That doesn't mean Brad has veto powers, but it does mean that the guy who will be dealing with and giving out playing time to the players has the chance to tell Danny who he thinks he would work best with/would help the team the most
Having an unclear chain of command is dangerous, but ignoring the opinions of those closest to the issues is just as (if not more) dangerous
Fictional scenario
Let's assume Danny Ainge was very high on one of Bitadze/Fernando/Gafford/Kabengele (player C) after intel from scouts. Then he asks Brad Stevens what he thinks, and Brad tells him he'd like to have someone who can replace Al Horford. After some discussions they come to the conclusion that (player C) is indeed very talented as a rim runner, but lacks the potential IQ/passing/versatility to be able to play a role like Al in Brad's system.
Austin Ainge mentions Grant Williams as an option, they have him high on their list as well. Brad doesn't know much about Williams, but there's enough video material to get him up-to-date and they have Williams in for an extra work-out and Brad likes what he sees. There are some short-comings, but Brad is convinced he can work with him on that. After their initial nr 1 option (player C), everybody is convinced there's little difference in talent so Grant Williams wouldn't be a bad pick.
Conveniently he's mocked later than #14, so they can get him at #20 (or #21) and use #14 to take a gamble on Romeo Langford. Another player that was high on their list. Now everybody is convinced and agrees with the outcome of drafting Williams and Langford.
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We don't know how much influence Brad has (knowingly or unknowingly) and if a scenario like this is even remotely plausible, but that it is even possible I find very disturbing. Management needs to dictate roster composition, drafting, trades, contracts and overall policy. A coach just has to deliver with the material he gets to work with. Just as a player (like Irving) needs to play according to the system.
I see literally nothing wrong with that scenario. Danny and the scouting team trust Brad, and value his opinion. Any functional organization has great communication from top to bottom. This just sounds like you not liking the picks they chose, and trying to find something or someone to blame.
I don't like the picks overall, you're correct, but I don't think for example that Grant Williams at #21 is such a bad pick.
But it reeks like the picks (Langford and Williams) were made mostly to accommadate Stevens' system.
I suspect that if we had a different coach our draft picks would be different too, and I have a problem with that.
I suspect that if we had a different roster, our draft picks would be different
Because, you know, context is important when it comes to draft picks. In the situation the team is in now, we should be taking the guys we think will have the best career; had the AD trade gone through and Kyrie/Al planned on staying, we would have been looking for at least one guy that could make a more immediate impact.
It's one thing if you're talking about a coach telling the GM to pass up a clear top pick because he'd rather coach a clearly less talented player (for personal or personality reasons), but that clearly isn't the case.