Duncan was the constant and most important variable. Not Pop's coaching, R.C.'s drafting/signing or even Tony, Manu or Robinson.
He was a franchise transitioning piece and the last two summers (since his retirement) prove his importance. I hate speaking in hypotheticals, but I seriously doubt LaMarcus and Kawhi pull this with Timmy still in the fold.
Also, Pop and R.C. knee they also had Duncan preaching the company line and setting the example.
They thought they had that with Leonard, but clearly he's easily influenced by internal (Simmons & Dedmon?) and external forces.
, Timmy was able to block out his ex - wife cheating on him, a financial advisor swindling him and a divorce while still leading the team!
He also played with a box of rocks for brains like Stephen Jackson (ala Simmons/Dedmon) and that didn't sour his relationship with Pop/R.C.