If a county wants better radio coverage or a simulcast cell, the county is going to have to fund that, and there probably isn't much cost difference between installing 3 additional towers and giving control to IPSC to add to SAFE-T; or installing the 3 towers and having it be it's own radio system that the county controls.
Except that if the county buys its own radio system, they are forever on the hook to maintain it. If they go with SAFE-T, routine maintenance is IPSC's responsibility. It's the difference between making radio system maintenance a line item in the county's budget that has to be funded every year vs. everybody in the county paying state taxes to maintain the state system (which we do, anyway).
Elected county officials are likely hesitant of spending 6-figures ($$$) on radio system infrastructure, then giving it away for the state to control.
Sadly, elected officials don't seem to be able to look past their next election campaign. In the town where I live, we are still paying the price of decisions made by elected officials years ago even though those officials are long gone from town government. It's up to the electorate to stand up in council or commission meetings and ask "what is the decision you are making today going to cost us 5, 10, 15 years from now?".
As far as radios themselves, if they're bought via grants distributed thru the state, I believe one of the stipulations is the ability to work on SAFE-T for interoperability purposes. If a county programs SAFE-T and a local county P25 system into the radios then they likely still qualify for most radio grants.
But, if a county buys into a DMR or NXDN system, they won't be able to use those radios on SAFE-T unless they buy expensive multi-mode radios or unless they plan on manually-initiated "patches" between systems. A "consultant" or vendor pushing a county towards DMR is isolating the county from SAFE-T and, possibly, neighboring jurisdictions. The elected officials need to know that, but it rarely comes up when the bids are opened.
As an aside, there was a recent police chase that started in Greencastle and ended on I-465 in Indianapolis. Greencastle Police Dispatch was seamlessly patched to mutual aid and other talkgroups accessible to Putnam County, Hendricks County, Morgan County, ISP, and IMPD. How would that have worked if Greencastle or Putnam County were on a separate, non-P25 system?
As DiGiTaLD, mentioned above, the whole idea behind SAFE-T was interoperability. Some counties, such as Marion, have built their own systems and carefully maintained interoperability with SAFE-T and surrounding counties. Is that always the case with these other standalone county systems?
Also, on a related subject, who is going to be financially responsible for upgrading the current county-funded simulcast cells from P25 Phase 1 to P25 Phase 2 when SAFE-T switches to Phase 2?
An excellent question. When SAFE-T goes to Phase 2, will IPSC allow islands of Phase 1 to operate on the system? I kinda doubt it. Is IPSC figuring that they will have to upgrade these county-purchased sites? I suspect that they are because once the county pays for the site, maintenance is an IPSC problem.
...the aging SAFE-T system...
I'm fairly confident that long-term maintenance and future upgrades are built-in to IPSC's funding plans. If they aren't, then we all have a problem.