Well, this certainly is no surprise as I would tell the same thing to an assistant on a coaching staff of my own. The Sixers are set up in position to have a sixty-plus loss season and are the favorites to win the Wiggins sweepstakes. If I'm a coach on a great coaching staff why would I just ignore it and let my fresh new top assistant take an offer and stoop so low to a loser franchise such as the Sixers? I would look out for all of my staff's best interest and find a way to keep my staff intact.
I can understand why the Spurs would urge their fresh new assistant to reject an offer from the Sixers. In that aspect, they're saying they're the San Antonio Spurs and we're the well... we're just the Sixers and there will be better options in the future. For a team to to want to hold on to member of a coaching staff who isn't regarded as a highly touted coach or future coach he sure does mean a lot to their staff.
If the Sixers aren't able to land the Spurs new assistant then I'm sure nobody would mind promoting three-year assistant Michael Curry who has worked along side of Doug Collins for his three-year stint with the team. Michael Curry is touted to be a defensive minded coach.
Promoting Michael Curry as your head coach is likely going to be a patch job. He will most likely be a one-year band aid. If that is the case, if you bring in Brown as your coach and have a dreadful season as expected, would you fire him after the first year? It wouldn't make much sense.
In a situation like this you would heavily look in hind sight to find a coach who will develop the talent and try to reel in some star free agents in the future if possible. My explanation of why the Spurs brass would urge their new assistant coach to stay with the organization is because he's extremely excellent in terms of player development. If we somehow manage to land the new assistant, we may be a better team for years to come.
Then again, there are some questions. Will he have an impact on the players? How is he in terms of coaching style? If he has a positive impact on this roster then the job is his. However, if Brown does indeed reject the Sixers offer to be the new head coach then the job will most likely go Michael Curry. Either way, it does not change the fact that the Sixers are inevitably one of the worst teams in the league this year