His option next summer is more intriguing. Full non-guarantee and until 7/15 to exercise it (well into free agency). Spurs could theoretically take back salary from a team looking to sign someone who needs max room - $7 million times 125%-150% depending on if Spurs are under cap, and then other team can release Diaw with Spurs taking back a player they want and/or draft compensation. Or, in if Spurs want to chase someone, they can decide to cut him then.
This year the savings are only around $4 million - he may or may not be worth that depending on if he gets into shape. Luckily it's an Olympic season.
You're assuming too that GSW and OKC continue to operate at such a high level. Bogut and Iggy will be a year older, and there's a chance Barnes and Ezeli both get stupid offer sheets. Knowing them though, they'll turn Ian Clark into a 6MotY.
Spurs have a lot more long term cap flexibility than either of those teams after this summer and the youngest superstar of the bunch. On the other hand, LMA could hit a wall a year or two from now.