A smallish guard with a history of injury at 33 doesn't represent a max value investment in the NBA, no matter how good he is. It is actually very likely that he will soon fall off a cliff. If they overpay, they do so on the basis that they go all in for next 1-2 seasons and try to win a championship here and now, while conceding that there were no better options. A 36 year old CP sure as is not leading them to the promised land if he can't do it at 34.
The good think for the Rockets is that they can explore other options because it's not like there is market for max contracts for CP, no matter how deluded he is. In fact the only teams who will do it are the ones that are so desperate to win now, that they are willing to overpay. And those teams are essentially either the Rockets or Cleveland, if they keep LeBron. And if LeBron were willing to take the riks of playing with CP3, than playing alongside Harden is a better option.
Sixers and Celtics are not ruining their future and flexibility for CP3, and no other team is close enough to take such a risk.