The FirstNet core was turned up earlier this spring. Turning up LTE Band 14 is in process, but that's going to take upwards of 5 years to complete.
AT&T is putting FirstNet on the existing LTE bands now, before Band 14 is fully rolled out. The preemption (Public safety users take priority over consumers, as in kicks them off the cell) will be turned up over the next year.
FirstNet is never going to cover 100% of the population or the country. Here in California, the plan is to provide:
99.8% of California's population.
98.7% of California's rural population.
77.2% of California's Geography.
Also, the FirstNet network is supposed to provide 99.99% uptime, which is good, but less than what a well built/designed dedicated system will do. Usually "Five 9's" (99.999%) is considered good for a telephone system.
99.999% up time equals about 5.26 minutes of down time a year.
99.99% up time equals 52.56 minutes of down time a year.
Either way, downtime sucks, especially for public safety, but comparing 5 minutes versus almost 1 hour is a big deal.
So, based on the geographic coverage and the "four 9's" uptime, it's never going to be a service that replaces a properly built and designed dedicated 2 way radio system.
Even FirstNet is saying "keep your radio system".
Likely some departments will want to try to jump to FirstNet for everything, especially when they provide Push To Talk service, just like they did when NexTel came to town, but it'll likely turn out the same way.
The other issue is that simplex communications (handset to handset) haven't been addressed yet. Even if they do, the handsets are pretty low powered, so it's going to rely heavily on the AT&T/FirstNet infrastructure.
What will happen is less voice traffic will be carried over the radio. As it becomes easier to send photos, video, Computer Aided Dispatch, and similar stuff over FirstNet, dedicated voice dispatching will scale back.
Another neat approach to look at is that some new LMR radios designed for public safety use will include an LTE module. That will let a two way radio that is outside the LMR system coverage connect back via FirstNet. If that takes off well, it's going to be an interesting solution for addressing coverage deficiencies...