I see this move as bad for Simmons in multiple ways:
HIS STOCK
This weakens his stock even further. It is yet another example of Simmons shying away from a more challenging, higher pressure option. He shrinks in the playoffs, and he completely avoids the challenge of the real world championships (or Olympics equivalent). It was an opportunity to demonstrate better FT shooting, because surely, the only way is up after going under 40% in NBA playoffs. It strengthens the argument that his mental strength is his real weakness. A major weakness. Not a leader. Not a franchise player.
HIS GAME
He and his team are dreaming if they think practicing free-throws in a gym will make the difference to his game. He needs mentorship. Sam Cassell may be helping, but he is still surrounded by a team and fans that are highly critical of him, which applies even more pressure on him. Surrounding himself with grounded, supportive veterans, in a new environment, a new system, surrounded by shooters, is worth exploring as an option. He might learn a thing or two. It may not be the solution to his flaws, but it might help and wouldn't hurt.
FAN ADMIRATION
In Australia, he could've redeemed himself. He could've become a Boomers legend. He's the best player -- no, most talented player -- Australia has ever produced. He could've built his brand in Australia and been a face of the Olympics. Instead, he pulls out late, again. He appears even less connected to Australia. He has become difficult to support. It's tough to find Simmons fans now, even in Australia. He appears so mentally weak that he is either manipulated by others (Clutch? 76ers?) to avoid the Olympics, or he simply evades making the call until he is forced into it, once again. In USA, he is already a laughing stock. In this off-season he will probably pop up on Instagram showing compilation videos of him draining 3s against scrubs. He will be pumped up, but it's all a mirage if he doesn't fix the actual problem with his game.