I disagree with anyone mentioning age, I don't think age will play into it. It's not like Hield is expected to fall off a cliff soon, his prime should extend through the length of this contract, 28-31. Maybe if most of Hield's contract was going to played out on the wrong side of 30, than ya, or unless you think Brown's realistic ceiling just blows Hield's out of the water, but I don't think either is the case here.
It's not hard to imagine Hield continuing to be a 20ppg, 40%+ 3p shooter for the next 4-5 years, which he already currently is doing. It's hard for me to imagine Brown far exceeding that over the same time frame (not talking so much about the stats, but the value/ overall production). It's easier to imagine Brown never getting there to begin with.
You're telling me if you had a 27 year old and 23 year old who played the same position. And the 27 year old was clearly better, and had maybe fringe All-Star potential, you'd want to pay the 23 year old more? Well he better have super star level potential then, which I just don't think Brown has.
Even if you think Brown will be the next Jimmy Butler, you let him play out this season and show you first.
I think this gives Ainge more leverage than Brown right now. If Brown signs an extension today, then I expect it to be roughly the same (give or take a couple of mil) or less, not significantly more. I guess we shall see though.