Maybe you don't understand what a double edged sword means in the context of a debate. It refers to an argument or piece of data that can be used for your side but can also be used against your side.
Antis can and will use that argument to suggest more gun control whether you think it makes sense or not.
Take regulation of any kind. You can point and say "look at that, regulations didn't work, we should get rid of them" while they can say "look at that, regulations didn't work, we need new/more regulations"
All your counter points can really just end up being ammo to use against you. They can apply the same logic and just reverse the argument by saying "we let people have some guns but that didn't work so obviously letting people have guns doesn't work at all"
The crux of the problem is a flaw in your logic. You believe that because A regulation failed that necessarily all regulations will fail. A failure of a regulation does not invalidate the process of regulation altogether. Thus, if a regulation fails you have two possible roads to consider, more regulation or less regulation and this is where the double edged sword comes into play.
There is a reason the 2004 Assault Weapons Ban was allowed to expire. There was no data to demonstrate the ban made any difference whatsoever in violence or deaths by the guns affected by the ban.
This shows that sometimes the Congress gets it right. They tried to enact controls, evaluated the results, and decided to discontinue the controls.
When you do nothing, and the results are the same as doing more than nothing, it's obvious doing more than nothing is a waste of resources.
Had there been a minor improvement in the statistics, then more control would have been enacted. If a little gets a little, then more will get us more. Since that wasn't the case, it was impossible to justify continuing the controls, much less additional controls.
Same for this situation. If the California rules are not stopping criminals from doing what these terrorists did, just like having no laws would have done, then there is no justification for doing more.
More laws will only affect the law abiding -- the same people following today's laws. New laws would be just as ineffective controlling criminals.
The conclusion is inescapable. There's no evidence existing laws prevented anything, now or in the past. There's no double-edged sword if the people considering more legislation have half a brain.
What you still refuse to consider is these people are not going to agree with anything that avoids gun control. They have an agenda. They are not interested in actual solutions short of an all out ban. Worrying about giving therm ammunition is a waste of energy, since they really don't need ammunition to push their agenda.