I think the reason they've been a "substantial problem in the past" is the past situation, (and was mentioned by another poster) for example speed traps were run often on CSP 3. And at altitude, sure, they'd hit a variety of sites. And if there was no PL, then those would repeat and the area blanketed is larger.
If a Helicopter, flying 1,000' AGL has a radio with a limited number of sites programed, and each site is looking for that authorized radio on a specific control channel, and they select a talkgroup close to them, there's not much likelihood that you're going to be hitting sites a long way away. Even as you fly along, there's still only going to be a specific control channel, looking for an authorized user, and carrying the conversation forward. If a helicopter from northern Colorado gets up in the air and wants to talk to an agency out on the eastern plains, it's not going to have the same control channel, and the site (for example) hit near Estes Park, won't have Milken Fire's talkgroup on it. They'd have to have, perhaps, Glenwood Springs Fire's talkgroup in their radio, and selected, to cause a problem with affiliation while traveling
The DTRS is not intended for a HEMS crew to call DHMC from Grand Junction (although I think you could), if used for a med report, it's when calling the New St. A's as they pop out over Green Mountain. The effect wouldn't be that much different than a West Metro unit on top of Green Mountain with an injured bike rider. And with the power output limited to 3 watts on a fixed unit in the aircraft, and a directional antenna with a downward pattern, as it's designed to work.
Besides that, how often is this going to be in use for statewide conversations? You're talking about an aircraft, communicating with a ground unit just prior to landing, and perhaps those departing Thank You's from each agency. Without direct communication, you're still seeing HEMS using FERN, NLEC, HEAR etc, while in the air, and if the juridictional communications center has FERN patched to their 800 system, and that's left open 24/7, I can see the potential from an airborne transmission on FERN going to MANY MANY agencies that have left the patch active. Isn't it LESS likely that having an authorized user, on a recognized radio on a predetermined control channel, on the agency underneath the aircraft's talkgroup, talking to an Engine on the ground during landing, have a more discreet conversation than blasting everyone on a FERN-to-800 talkgroup?
I'm just wondering how much of this is a byproduct of not understanding how DTRS works, history from a system that's not designed for aircraft, and the "turn off your cellphone or we'll crash" FAA hype.
All food for thought