With an inexhaustable supply of taxpayer funding, there is no incentive to centralize operations at any level of government. JohnSC has it right, but common sense doesn't generally factor in to government operations.
At the very least, it would seem that Greenville County Fire Departments could choose a single band to use - VHF or UHF. UHF seems to me to be the better choice, as it would allow interoperability with Greenvillle EMS and GCSO, who also use UHF. Those still using VHF radios would be the ones to change. With narrowbanding and the number of nearby PDs moving to 800, it would seem like UHF frequencies would be abundant. That would eliminate the need for multiple radios.
Every FD could still have "their" channel and keep "their" dispatcher if they so choose, but being on the same band would allow for some common channels to be used at larger incidents.