I think it is a pretty fair offer at this point, and Brown wouldn't be wrong either way, in terms of either taking it or declining it.
Benefits of Signing: Has 80 million guaranteed; set for multiple lives, no worries; takes him through his age 23, 25, 26, 27 seasons; all set to sign a Max Deal at a higher scale right at his likely peak. Avoids the potential of regression/stagnation causing lower offers later.
Benefits of waiting: "Betting on himself;" keeps open the chance of snagging a max deal.
For the Celtics:
Benefits of offering 80/4 now: Lock him in at below max if he continues to improve.
Downside: Blows out his knee in year 4 after contract is signed; never improves.
What I generally believe is there is basically no benefit for the Celtics to offer a max deal now before next offseason. That chance will always be on the table. If he doesn't take a moderate discount, fine; if he plays like a star this year, he will still be an unrestricted free agent with Celtics having the right to match any offer, max or otherwise. The only downside would be if Jaylen is so offended at not getting a max offer, that, to spite the Celtics, he just takes the QO to play a 5th season for the Celts then become a UFA just on principal.
Personally, I hope he has a season that "earns" him a max contract offer in the next offseason.
I am mildly worried about him in terms of becoming a "STAR." I haven't seen DPOY level defense, though he can continue to improve. PER (flawed, I acknowledge) was pretty stagnant last year, a significant level below that of typical "stars." I worry his 3 point shooting is a bit fluky given his consistently mediocre free throw percentages. Hope I'm wrong and he takes it up another level!