You think a simple cable lock would stop a 20-something who could use a computer and manipulate mechanical equipment (an AR, for example)?
Beyond the fact that, as a practical matter, cable locks would do nothing to deter a halfway competent thief, there is another issue - why should the owner of a gun be held to a higher standard than the owner of a car?
If a person comes onto your property, finds your keys, and steals your car, the DA does not charge you with failing to secure a motor vehicle.
Cars kill a lot more people every year than do guns.
Personally, I can afford (and have) a pretty solid gun safe. I believe in securing weapons not under my immediate control. I do not believe that failing to do so should make me responsible for the actions of a thief, legally nor morally.
And I think you are far too eager to appease unreasonable people, who ultimately cannot be appeased.