Your fear seems to be that he will play too well and we will have to spend money on him that then can't be spent on someone else at the risk of him walking away and leaving us with nothing when he hits free agency. The way I look at it is that if he plays well, then we have an asset that can help us. And if he returns to All Star form and we extend him then we have an asset that is worth more than it was at the start of the year, which we can then either keep or trade elsewhere. The alternative would be for him to play badly, underperform his contract, be of no help to us this year and then become difficult to offload.
If building a long term window is still part of the goal, maintaining the stores of assets is imperative. Hayward simplifies any star transfer relative to any other trade package the Celtics can offer. It makes taking a risk more palatable so the future isn't spoiled, as was a huge concern with recent attempts to bring in stars. My fear is that Hayward can no longer be a trade asset when the Celtics make their move. The timing is the concern.
This season the potential pool of available stars primarily consists of Blake Griffin, Kevin Love, Bradley Beal, and Andre Drummond. Not bad names but I vividly see them not answering the holes the Celtics need filled. Someone else midway through the season could become a new source, but my impression is that options following this season would be more fruitful.
This means I want him back for at least the next season. The two options are simply he opts in or the Celtics re-sign him to a three plus one deal for a lot of money. The hesitation to commit is rooted is how it changes his status as an asset. In my opinion, it could ruin the chances of using him in trades, which is why I prefer him exercising the player option. Unless a team trading their star wants to stay competitive, instead of rebuilding, Hayward, at 30 years old, without an expiring deal no longer facilitates what that team is searching for. Any limitation like that would make him a central part of the Celtics near future, which isn't a bad thing, but making the necessary upgrade would shift the responsibility solely to numerous loose and cheap, young assets, supposedly keeping open the Celtics future extension of any window.
To your points about his success. Hayward succeeding, as you said, is good for being an asset. His failure, likewise, is bad for him as an asset. How that fits to my line is not entirely clear. The defining line is unlikely to be binary, as others have pointed out. Playing somewhere in that spectrum is likely to happen. At what point does he bet on himself by opting out? I'm starting to think this is likely as it negates the Celtics' control over his future.