Tanking with the expectation of drafting another Duncan is stupid. But in the Salary Cap Age it's not enough to have an exceptional player. You also have to be able to afford to keep him and still afford to surround him with talented support players (ask Cleveland). And that means you almost have to get a talent infusion through the draft once in a while.
The Spurs had the best winning percentage of any team in any major sport for two decades. And that means the lowest draft picks out of every team in the league. They drafted very well, considering. But it's undeniable that other teams picked up exceptionally talented players on cheap rookie contracts during that time. And they were only able to be successful doing that because Tim really was a generational talent (and, yes, Tony and Manu). That's been over for several years, and the team has diminished each one of those years.
They desperately need the kind of talent infusion that they can only get from a high draft pick. There's not a Tim, or a Kawhi, in most drafts. But the talent gap between the first few picks and the rest of the draft is generally pretty severe. So, no, you don't tank with the expectation of landing the next Duncan, but doing a 1-year rebuild that includes a high draft pick is a necessity now.
The year after Tim retired, the team should have done a 1-year mini-tank with the idea of picking up a Jamal Murray or Buddy Heild. If they had done that, they would be a much better team right now, for a number of reasons. They wouldn't have given out those two contracts to Pau and Patty. They might even still have the Autist on the team. That opportunity passed, but they still need the talent infusion of a Top 10 pick.