Arguments off the top of my head for leaving guns alone/loosening some gun laws-
The "Red Dawn" scenario- some country catches the military off guard and invades the country with every intent to rule. People are gonna want to fight back themselves to protect family, home, and way of life. Hard to do that without guns to take out of the closet and dust off. Cons: highly unlikely scenario.
Oppressive government scenario: attempt to use soldiers to subdue/pacify a state/region/etc, take away rights, etc. Gotta be able to make a stand, I suppose. Cons: Does the civil war ring a bell? Lots of dead people, all American. Also, soldiers vow to uphold and defend the constitution and are obligated to have the moral and personal courage to stand up against something that just isn't right. There would probably be a lot of conflicted and unwilling soldiers in this scenario.
When you never know who's packing, you give everyone that much more basic respect.
Not that there won't be a lot of hot-heads doing stupid shit and getting themselves and others killed. That happens with or without firearms, so that doesn't really change.
But mainly, punishing the masses for the stupidity and wrong doing of a few is really stupid and ABSOLUTELY foments discontent and unrest, causing trouble in the first place. As a military guy, I have first-hand experience with this bullshit.
The 'bad' guys are gonna get their hands on prohibited stuff anyways. That's part of why they're bad guys. Now you're taking away stuff from the vast majority of people who haven't done anything wrong and won't do anything wrong with firearms, handicapping their ability to protect themselves. There are always people who can handle certain things and responsibilities, and others who can't.
I really don't like the NRA, but I've come to accept that fact there are some things I do agree with them on. I'd probably write more but I don't want to spend that much more time on this one topic.