Because you are putting WAY too much stock on Splitter who has missed over 50 games the past 2 seasons and the spurs were still able to sustain a high level of defense. Will it hurt them a bit at the rim? Sure, but you are putting way too much emphasis in a position in where the top offensive bigs in the league are either inconsistent or don't have a good enough team around them to compete. Russell Westbrook will give them the most problems because Tony Parker simply cannot contain him and he drives to the basket hard. Harden flops in the playoffs so I doubt the Spurs will worry about him.
What the Spurs have to worry about is the perimeter defense first because all the great teams in the West are stacked at the Forwards and Guards. Marc Gasol will probably feast on them, but the Grizzlies are not good enough to beat them.
Dwight Howard is not consistent enough on offense to worry about.
DeAndre Jordan will be taken care of, get rid of the alley opportunities with him and foul when necessary.
Anthony Davis is still 2-3 years away from the Pelicans building a solid core around him to really threaten anyone.
The Kings are not making the Playoffs, so they don't have to worry about Cousins.