Shaq was past his prime for sure but his biggest issue was always his motivation.If he were motivated and dedicated, he was still leaps and bounds better than other centers and could've won more championships.
The Lakers had to make a tough choice but given that Shaq was demanding a $100m extension, they had no other options.
Sometimes, it takes a change of scene to get someone to refocus and that's what happened to Shaq. He came to play in shape, first time in 3 years. And he willingly accpeted being the lesser part of co-star to Wade, something he could've done with Kobe.
He was a big part of that championship run, second only to the most outrageous calls for Wade.
With the way breathing fouls are called in Wade's favor, it ain't too hard to "carry" the team.