So the Celtics can take Hayward into their cap space with a max or near-max offer. They can then turn around and trade for Paul George. The problem arises when they ask Paul George to sign an extension with the Celtics rather than become a free agent next summer. Paul George can only be extended starting with a 20% raise beyond his current salary, which would end up around $7M short of his max salary next summer. The Celtics' next move would be to clear more cap space so that they can use it to raise PG's current salary, such that an extension gets closer to his max. And to do that, the Celtics would have to trade out a couple more players, thereby reducing their depth somewhat.
The Celtics may be willing to gamble on Paul George without an extension. They may feel they can convince him to stay with Boston at the end of next year.
However, the pitch to Hayward is that he can join a superteam in Boston including Paul George. If Hayward signs and Paul George bolts to LA (like he's said he will), then Hayward is left high and dry with an aging IT and Horford and a couple young players. In other words, Boston would be asking Hayward to take the risk that Paul George signs and extension or re-signs with Boston. That's a risk that Hayward doesn't need to take.
The Celtics may be willing to gamble on Paul George without an extension. They may feel they can convince him to stay with Boston at the end of next year.
However, the pitch to Hayward is that he can join a superteam in Boston including Paul George. If Hayward signs and Paul George bolts to LA (like he's said he will), then Hayward is left high and dry with an aging IT and Horford and a couple young players. In other words, Boston would be asking Hayward to take the risk that Paul George signs and extension or re-signs with Boston. That's a risk that Hayward doesn't need to take.