I think the issue with that scenario is that some hack of a Mac in developer mode will get reported in the media in big scary headlines as "All Macs finally hackable" - (which would technically be true but it'd be missing the fact that developer mode would have needed to be enabled). And then the flood of Grandma voicemail messages would startg.
A single news cycle of a story full of misinformation would dissuade them? Sorry it has to be a far bigger problem than that. A very complicated easily communicated opt-in hacking risk is very very low on the totem pole.
