You may not have noticed but the output power was expressed in ERP. An ERP of 100 watts corresponds to an actual radiated power of 5-10 watts, depending on the type of antenna used. Ham classes plus an online calculator...
No. I think you may be misunderstanding the way it works.
ERP = Effective Radiated Power
ERP takes into account transmitter output power. It subtracts duplexer losses (if it's a repeater), feedline losses, and anything else in the path. It then multiplies that by antenna gain.
The wattage of the transmitter could be anything, antenna gain can boost the ERP by focusing what power gets to the antenna in a specific direction.
An ERP of 100 watts would equal:
A 100 watt radio right at the antenna into a unity gain antenna.
A 50 watt radio right at the antenna into a 3dB gain antenna
Etc….
I live in the mountainous Temecula area that includes the Cleveland National Forest. 5-10 watts at 800mhz is not going to cover more than a couple of miles. I'm sure in more demanding areas like the 58 going from Mojave to Bakersfield their HT's would stand no chance of being heard on the station channel. Or in Forest Falls in the San Bernardino Mountains where I was hiking until the fires. The Arrowhead station is a very long ways away. I'm sure you didn't intend to, but you write as if the station frequency is a 700 MHz version of their low-band system. I get that from your statement that they're transmit power is ~100w. That would imply a wide area coverage. That it is not. According to the CHP radio instruction guide, the real purpose of this frequency is for VIP escorts through the area. It is intended more for car to car then station to car.
You are correct, 5 to 10 watts isn't going to work very far in most instances. However, it can under some conditions. Straight line of sight it can. I run a few 800MHz systems, and I can easily hit those 70 miles south if I have a clear path with a 15 watt radio, and if careful, a 3 watt portable.
But as you pointed out, in the mountainous areas of Southern California, that's not going to work reliably.
The low band traffic could easily be patched to the 700MHz channels. That's what happens on the mobile repeaters, and it's likely possible at the stations. Not sure if they actually do it, though.
The only question remaining is the original one. Will they go to encryption?
That is the $64,000 question. So far I don't think anyone has any proof one way or the other. If you can find something that says they'll encrypt on low band, I'd enjoy the read.