Every criminal was once law-abiding until they weren't anymore. Just because someone may be following the law now, it doesn't mean that they are as safe as they should be, as proficient as they should be, or has the full mental capabilities to remain law-abiding. Now, we may not be able to do all that much about the last one, not without an overhaul of our mental health system as well as mental health background checks to go with that, but we certainly can make sure that people are safer, trained better, and perhaps, just perhaps, limited in the types and/or amount of firearms they may have. That is to say, there is no rational legal reason for people to stockpile military grade weapons in their home. That right there speaks to a mental health issue (paranoia, conspiracies,...).
The idea that everything is fine as is and doesn't need, or at least could use, improvement is part of the problem. If someone is "law-abiding" as you put it, if they are responsible with guns and are not part of the problem, then why should they mind procedures being put in place to insure everyone else is as responsible as them?