Yeah, any chances of extending Brown early ended when Devin Booker signed for 40MM a year.It was 5 years, $163 million but making all-NBA pushed it up to nearly $200 million, I'm pretty sure.
As for Brown, I'm not saying it makes sense for him to leave, just that he won't likely sign an extension. Boston will have the edge in terms of years and dollars it can offer, but he will still become a FA, so there's more risk he leaves than if they could just lock him up on a max extension tomorrow.
Edit: if they existed at all before.