Kendrick would have to be murdered in a shoot-out with police or release a conscious album while serving a life sentence while giving classic interviews from prison to "surpass" 2Pac.
2Pac is essentially hip-hop's Jesus. He died in his prime and left his message to live on like a religion.
His martyrdom created a huge bias and there's almost a subtle air of disrespect when you place any living artist in the same genre above him. So even if many eventually feel that Kendrick's music is better, he'd have to have something huge happen outside of music to ever make a real debate out of it.