OTOH, unless we're signing Hayward, trading him makes no sense. The arguments for keeping him are the same as trading him, he has value because of his contract and because he is a solid, tough minded role player who is an important cog.
Sounds like a good argument for not signing Hayward.
But Crowder is more than "a solid... role player". He's one of the top wing defenders in the league, and one of the top shooters in the league.
Hayward is a shot-creator, while Crowder needs other guys to get him the ball, mostly. Gordon's skillset is coveted around the league, and Boston needs someone who does what he does.
So... will Jaylen Brown give you some of that? Yes, clearly that's in his future, with more trips to the line and stopper defense.
Will Markelle Fultz give you some of that? Obviously.
Question is, is Boston willing to wait while those guys develop their games? I'm not sure the argument that GS/Clev are unbeatable for the next couple of years is as good as it sounds - but there is a lot to be said for playing the long game.
People have questioned Blake Griffin's reliability because of his injury history. That's hard to evaluate - while he doesn't have a chronic problem in some part of his body, perhaps he's just one of those injury-prone people (I think that there are people like that). However, his list of injuries does not really seem to follow a pattern, I'd say.
Given the talent in the pipeline and the current success of the team, I think that Griffin is a good bet - worth the risk, whatever it might be, of his playing 60 games next year. He's a triple-threat, great passer, ups the defensive rebounding, fills a spot. The Celtics have a lot of strength at the wings already but few guaranteed contracts for the bigs. So Blake solves three immediate problems: shot-creation, defensive rebounding, and the roster need for a big. In my view those are the three most urgent things the team needs to address, and you can solve them by signing one player. Hayward only solves one of them, and creates an unworkable logjam.