I'm not sure I'm seeing this "precipitous" drop-off people are talking about.
Looking at Tim's offensive stats:
1) His true shooting percentage is down a bit from last year, but higher than '13-'14, '11-'12, '10-'11, '05-'06, or '03-'04. In fact, it's just .007 points lower than his career average, .544 vs .551.
2) Per-36, his scoring average is down to 12.3 pts, but that's on just 5.1 shots. So his pts/shot is 2.41, compared to last year when he was at 2.54. So a bit of a drop, but nothing crazy.
3) His usage is the main thing that's way down, and that's likely an intentional move to defer to LMA and Kawhi. He's down to 17.3, the only time under 20 in his career.
4) Instead of using that energy on offense, he's using it on defense... hence, leading the league in DRPM, and topping all centers in RPM.
I think Tim is very intentionally shifting his role, to still feel relevant in the new Team's structure. Not unlike what David did with him. And since he wants to play "till the wheels fall off," I think that his ability to be successful in this new role (which clearly he is) can really help him extend his career another year or two.