I tried to do a pretty careful calculation of an extensions vs. a max contract with the Celtics vs. a max contract with a new team. It is hard to get to apples to apples because one contract would be 3 years, one 5 years, and one 4 years but I normalized by looking at only the first 3 years:
Extension: $119,687,814
Celtics Max: $129,856,000
New Team Max: $126,100,000
The extension assume the "likely" incentives and starts at about $36.7M, which is 120% over his last year of the current contract. The extension could be a little higher if he achieves his unlikely incentives (MVP, All NBA). The two Max contracts start at $40M which is based on something I read as the expected Max number by then (but this is just a guess). The extension and the Celtics Max increase by 8% per year and the Regular Max increases by 5%. The bottom line is that after 3 years, he would be about $10M in the hole taking an extension over a Celtics Max deal and would be about $6M in the hole over a regular max deal. This is not exact as there are assumptions but I feel this is a pretty fair estimate.
With the extension, he would be a UFA in 2027/28 and probably could sign for more than what even a Celtics max would be so he would start to make up the difference. At that point he could sign up to whatever the max is for a player with his tenure. This would be (or at least is expected to be) more than the 4th year of either of the other contract options. I was under the impression that the extension would be a lot less but after looking at the numbers, it is closer than I realized after the 3 years and he may well make that amount back up over the course of his next contract.
He may still chose to take this to UFA but he can get a pretty good deal in any case.
Okay, but again there's so many problems with this thinking:
A) If the cap is higher in 24-25 then the NBA is projecting right now the difference between his max contract and the "right now" extension only grows.
B) Incentives are nice, they aren't guaranteed. One reason for an extension is protecting against injury and if your salary includes games played, all star games, team success ect as Jaylen's does that removes some of the incentive to sign an extension.
C) This analysis ignores the supermax possibility, if Jaylen plays well and c's win 60+ games he will have an excellent shot at that massive extension. Lets say that 24-5 has a cap of 136 (currnet projection). Extension would start at 36.7, max would start at 40.8, supermax would be 47.6. Thats a massive difference.
Again, not saying there's literally 0 chance he resigns but there's a reason literally every analyst you hear talk about it says its unlikely.