The sharkfin is usually only for external comms. There's still other comms on separate antennas like Bluetooth that can be potential entry points to the vehicle.
Worse, even the external comms systems are moving to more redundant setups to mitigate signal loss scenarios.
I'll have to dig up the post but a gentleman with a relatively new Subaru was kind enough to share his explorations and found significant logic parts integrated into the sharkfin, not just the antenna bundle. Likely a cost measure.
How well does this extend to other vehicles? No clue.
All of that comms stuff has its own logic associated with it that usually lives up there in my experience. In the past, manufacturers have tried to avoid putting much stuff above the headliner beyond roof windows and speakers. It's hot, narrow, and vibrates a lot (especially in the center). Frankly, the antennas are only up there because antenna placement is very expensive magic that doesn't deign to obey the whims of mere "designers".
Things are changing though. Autonomous vehicles need large numbers of sensors up there anyways and you can't keep shoving everything under the cabin.
How available are full vehicle wiring diagrams these days? Not any good for radios built into computers like the bluetooth antenna in the head unit, but it might be nice to snip the wires to the sharkfin or other remote comm modules without having to tear at body work or computer modules.
Worse, even the external comms systems are moving to more redundant setups to mitigate signal loss scenarios.