That's a debatable point about the inability to overthrow the government by small arms. As previously mentioned, the military would be likely to fracture in a major social upheaval.
But even accepting the premise, the conclusion, it might as well be banned, doesn't follow. Why not, so the Right might as well not be infringed (because the government has nothing to fear)?
Oh yeah, it's all about "safety", right?
"Might as well" be banned means there's no difference whether they are banned or not under that hypothetical scenario.
Again, if you are going to hold presidential assassins out as examples of why the individual citizens' right to bear arms needs to be preserved, you're going to lose the argument all day long. It leads one to question whether the benefit of killing a hypothetical tyrant is greater than the cost of losing some of our democratically elected officials.