What are you talking about? Luffy read the number wrong, because he is an idiot. It was a gag.
If decreasing bounties would be a thing, Oda wouldn't tell us thru a gag, where the person can't read properly lol.
There is also no reason to decrease a criminals bounty.
Rayleigh said to Kizaru if they would throw away his bounty, he could life piecefully and Kizaru answered with - The sins of a pirate don't disappear.
If there is a mass murderer would you also lower his bounty because he stopped killing people or doesn't kill as many? This makes no sense.
Don Chinjaos bounty didn't decrease either, even tho he lost his most powerfull weapon.
I guess you really don't understand how bounty system work right?
bounty is a DANGER for World Government
underlings/or who one might serve
and personal power
all are part of bounty and therefore danger of a character
so OF COURSE a man like Dark King or maybe Marco LOST many of that danger when they become a single man rather than a important figure
Oda himself in sbs point that a YC have high bounty CAUSE he works for a Yonko
so when that Yonko dies and they are no longer part of a titanic crew there is no reason to have the SAME bounty
it's ONLY LOGICAL for a bounty to drop it can happen irl ... it have NO REASON not to drop in one piece world
Oda himself is a fool if he don't do this ... while he might very well implied it