2.) The starters will have one less defensive liability
While Johnson is a consistent contributor from a scoring perspective, his lackluster production on the defensive side of the ball is his most notable flaw. The entire roster was a disaster on defense a season ago, but many considered him to be one of the most significant liabilities on the team.
You have to consider the position he was in. The increased offensive responsibilities across the board hurt his effectiveness as a defender. When injuries left the Spurs shorthanded, Johnson matched up against some of the elite scorers in the league. For the most part, Keldon held his own when San Antonio was healthy, but there's a lot of room for him to take significant steps on this end.
Is Keldon an outright terrible defender? No, not at all. Johnson always brings defensive intensity on a nightly basis. However, he must offer more than a relentless motor to build a reputation as a serviceable stopped. The advanced numbers weren't kind to Johnson last season, and he had a 121.4 Defensive Rating that ranked 525th out of 539 players. Improving in that department should be a priority.
The Spurs preached defense all summer, but they fell flat in their season opener, allowing Dallas to score 126 points. Keldon was far from the only problem, but there were still issues with his off-ball awareness and inability to contain his man off the dribble. A move to the bench could take pressure off Johnson while bolstering the starting unit with a reliable point-of-attack defender like Tre Jones.