I think there's only one person who can answer this question: Gordon Hayward. Even at that, I bet Gordon doesn't know yet either.
The issue is: Why is he opting out and taking less money?
Speaking purely from a financial viewpoint: He would opt out in order to take a contract that pays him more over the duration of the contract than he might expect to earn if he opted-in for next year and _then_ got a new contract.
So, lets assume he opted-in ($34M) and then became an FA in 2021. He would be 31 years old and in the midst of a boom of competing free agents. The 2021 summer is expected to have a lot of big name free agents. Depending on how the rest of this an next years seasons go, it's hard to say how he will rank in that market and whether or not he could still command a 'max' contract. There will be more teams with money, but a lot of it will go away fast to a lot of the big names expected to be available.
His agent's job is to try to anticipate what his most likely as well as most pessimistic and optimistic scenarios would be. But let's toss out a
totally arbitrary number of say, ~$25M per over 3 years at that point as what he expects to be able to sign for. That would mean that his expected 4-year earnings starting next year would be ~$109M.
So, to get him to opt-out, you would need to offer him more than that over that period. For example, you could offer him a deal starting at $28M, with raises that drops his number for next year and gets him a total over that value total by the end of the 4 years. There are other possible pay schedules.
The trick is trying to anticipate what his earning power might be as a FA in the summer of 2021 versus what he might get as a FA now, in the summer of 2020. This coming summer is going to have almost no big name free agents. Fewer teams with max cap room, but enough where someone likely would want him at a max, assuming he continues to play well.
I personally think that some team will offer Gordon a max offer this summer and that he will opt-out and sign a new multi-year. I just don't know if that will be the Celtics. I think the Cs
can afford to do that and imho probably
should do that. I just don't know for certain that they will.
Also, while I believe his preference will be to stay, I don't think he should 'take a home-team discount'. No player should be doing that (unless maybe if also given a no-trade clause).