Smart is fine at PG (solid setup guard) but he needs more help from his two wings in terms of secondary ball-handling and playing. Tatum and Jaylen leave a lot to be desired in those areas.
That puts more pressure on the PG to do more with the ball and Smart is unable to handle that extra offensive load. He needs his wings to help share the load.
Matters would be made even worse by a limited offensive player like Baynes clogging things up.
Tatum and Brown are still very, very young players and so far have been asked to remain disciplined secondary scorers. There are a number of explosive NBA scorers that played lesser roles their first few years. Neither guy is a great iso/dribble drive player, but both have the basic tools to be great at it. They both need to tighten their handle, recognize rotations more quickly and develop reliable counter moves.
You get from here to there by reps. They are advanced enough to start taking on the responsibility, and the team needs them to make that leap. Its time to turn over the keys to the car. It will probably take the better part of a season for them to start playing that way efficiently, but we are going where they can take us, so we might as well commit to it.