Most countries ban guns for everyone, with that being the primary reason. It is not a fragile argument, it is simply a different weighting of values.

When I say “ban”, I mean “heavily restrict and track”, which is how I gather we are using the term “ban” in this context as well.

Well, "ban guns for everyone because some bad guy might get his hands on one" has always been the law in my country, and it has worked out extremely well. There has never been a mass shooting here, ever.