The thread argument is silly because the starting 5 will be fluid. The most experienced players most likely start in the early season with an exception or 2 for individuals that really step up or dominate in the off season. The starting five 1 or 2 months into the season will look different than on day 1. There's not enough separation between players 2 or 3 through 10 this year (unless James amazes).
It's possible that Johnson gains a ton of confidence by starting. He may not be nearly as effective coming off of the bench. With Bowden's experience he is more suited for entering as adjustments are implemented. Johnson might do better with a known plan that's been worked on for several days. Bowden can be put in when it's not clear where he's needed until the game has begun and can easily adapt.
Bowden maybe isn't nearly as dependable as we hope. When the attention of the defense and game plans shift from Grant Williams to the perimeter, it's kind if an unknown how he'll respond. Bowden could disappear at times. He has in the past. He hasn't shown to be the most consistent performer, although he has expanded what he does. Early on about all he was good for was to catch and shoot 3s. Early last year he was the backup PG, but he really didn't fill that role real well. Turner, another natural 2 guard, worked out better as Bones' backup. Of course it could have just evolved like that since Turner missed so much time until well into the season and Bowden was no longer needed as a PG once Turner was back on the floor. Bowden has improved significantly year over year over year, but I don't know yet if he can be depended on to be the go to threat.
Also, if the perimeter starts out a game playing poorly, there's not necessarily a lot to go to if Turner and Bowden are already on the floor. This board goes into caniption fits when time outs aren't used to disrupt bad trends. Bringing Bowden off of the bench would be more effective.
Bowden makes a better 6th man than Turner because he can be plugged into more spots. I like having somebody that potentially brings instant offense off the bench.
Don't confuse 6th best player with 6th man. There's not an award given out for the 2nd, 3rd, 4th, or 5th man of the year. Don't confuse starting with getting the most minutes.