That's the only realistic & cost-efficient approach given the threat model.
Edit: I am not being sarcastic at all. It will be exceedingly difficult to get rid of all the guns in the US so suggesting disarmament is a non-starter here. That means that terrorists will be able to get their hands on guns if they desire. Which means that you need someone to counter them with something. The logical conclusion is another person (preferably many people) with guns also. And since the police ratio in the US is on the order of 0.1% that's not going to give you an adequate response time as we've seen from mass shootings.
The real question is "would the cure be worse than the disease?" and while I personally think the answer is "no" that doesn't make me right. And given how much the left hates guns, I suspect that we'll never actually find out, so don't worry.
Power needs to be attacked, otherwise it's protection racket wouldn't work.