Bams defense is elite and his skill set leans heavily on his defensive impact, but his limited offensive game makes it hard for him to be the kind of player you can build around in a league so focused on scoring versatility
He's a great complimentary player to set screens, act as a hub for DHOs around elite perimeter creators on offense, catch lobs and occasionally get to the rim but that is as far as it goes with him on that end
The Bam/Herro core, is arguably worse than the Mitchell/Gobert core that Utah blew up a few years ago
Why are the Thunder doing this? They are getting one of the best versatile defenders in the league, making them absolutely elite on that end and with SGA/JDub they have two elite offensive talents to play around Bam as a facilitator on that end, they become immediate favorites to win the title for possibly years to come.

Heat would trade Butler as well and look for a reset in this deep draft.
They keep Herro and get him a defensive oriented back court partner in Wallace, then reevaluate on Herro based on his play in the next few years before signing him to any extension.
Certainly an unlikely scenario, but I have this as a valid path for them to look for a quicker timeline of contention than hoping to land another 2 way star around Bam/Herro and Herro also significantly improving into at least a top 10 offensive talent.
(Bam can be traded before deadline starting from 6th January btw.)