I thought you said you negotiated deals for a living?
What agent is going to have his client play out most of the year, risking injury, and then take a deal instead of testing the market where he is likely to have multiple suitors and get a higher price? If Brown isn't extended before the start of the season he will likely test free agency and then be signed by another team.
Which is why he won't sign this year. He will only sign this year if he we offer the money his agent expects to get in 2024 as I've already said.
This "sign early but cheaply for security" thing doesn't exist. Why would Brown sign now coming off a 700 yard season where he missed game in a crappy offense unless you offered him $20m+?
You think he's going to say "Thanks for the $15m for 3 season offer. That gives me financial security for my none existant family and insures me against the 1% chance of a career ending injury".
Or do you think he might want to put up 1000+ yards next season and try maximise what he gets paid?
This idea that you can sign a guy this year for $15m to save paying $20m next year is a myth. Brown could get $15m next year is he missed half the season.