This is the problem with the overall gun conversation, and I think centering on mass shootings, while still logical, doesn't help the issue. People seem to think there are categories of people. There are good, law abiding people. And then there are evil people who will stop at nothing to do damage to others. They'll break any law. How could they not? They're monsters.
Well, the overwhelming majority of gun violence and gun deaths isn't quite as black and white. People are law abiding until they're not. They're fine gun owners until there's a heated argument and then they're domestic abusers with access to a lethal weapon. They're fine gun owners until work has them down and then they grab a gun. They're fine gun owners until they're in the throes of depression and then that gun is a way out for themselves. That's the majority, and taking away the easiest, quickest access to the most damaging weapon mitigates that. I think you can see why a lot of these people might not "stop at nothing" to do what they're doing. People argued the same thing when it came to fixing the golden state bridge suicide issue. They thought people would just kill themselves another way, and reality said they didn't, not at the same rate. And if they did try they weren't as successful. That's what statistical mitigation of human psychology does. Realistically little things stop us on the way to everything all the time. It's human psychology. It's reality.
And that doesn't mean that mass shootings won't be helped by this. They will still be mitigated. Perhaps a few really might have been intent on doing damage, like you said, but we know through evidence that less of them will attempt it and that how they attempt it will be far less damaging than amassing 10+ firearms and shooting from a safe distance. The truly evil and hellbent will be mitigated, too. It won't be cut 100%, but instead of one of these massive events every year or two maybe it'll be one every 5-10 years. That's still a lot of lives just zeroing into mass shootings.
In addition, as
David Frum says in his piece the other day (which I think others should read):
It isn't necessarily that we'll catch them doing the big bad, it's that with a culture that doesn't see guns as every day objects, with controls in place, we'll catch him doing something else at some other point. At that point maybe he'll be locked away. Maybe we'll have a psychologist that can see him. Maybe we'll just be more suspicious of him. Either way, that's how this works. It's about statistical mitigation.