If you add Murphy the 28-29 cap is a mess. Without Murphy and assuming Steph continues on the max contract trajectory he is on and the Spurs sign nobody else due a salary in 28-29 and draft nobody, assuming yearly 7% growth in the cap, the Spurs 28-29 cap sheet looks like:
FOX: $59.2 million
WEMBY: $57.4 million
CASTLE: $47.4 million
VASSELL: $27.0 million
HARPER: $17.2 million
KORNET: $9.4 million
TOTAL (6 players): $217.6 million
CAP: $189.4 million
APRON1: $240.0 million
So you have about $22.4 million to fill out half the roster. Murphy costs $31 million in 28-29. Even without getting Murphy the Spurs are letting go of one of Fox, Castle, or Harper for 2029-30 unless the CBA changes. Make that trade and the Spurs are probably letting go of two of Fox, Castle, Harper, and Murphy for 29-30. So four picks is a lot to give up there IMO when those are really going to be needed to add talent on bargain basement money.
If I'm trading for Murphy it has to be for Vassell, matching salary, one good first (eg the ATL ones or ones with swaps), and one bad first at max. And that's a lot the way Vassell is playing, but it's the only way to make it make any sense whatsoever on paper.