With Gasol at the end of his run (has been for the last 3 years, IMO), and Aldridge being somewhat uncertain as to how long he can stay dominant, the Spurs definitely needed a guy who could be the future big man for the team. Poetl is as good as any of the better young centers in the league, and fits the Spurs system really well. I can't imagine they'd of turned down a Siakam, but they may have lobbied for Pascal and then "reluctantly" accepted Poetl as a counter-offer, knowing they coveted him. Certainly could see that strategy in this deal.
With as little bargaining power as Kawhi left the FO with, with his trade demands, I think folks are not giving the Spurs enough credit for what they got in return.