With respect, you are incorrect. The Founding Fathers proposed and ratified the 2nd Amendment for one purpose, and one purpose only: to have a final failsafe for our Republic in the event that the Constitution failed and we ended up with a tyranny. In other words, they wanted to be certain that the people had the means with which to overthrow any future tyranny; they also had the hope that a very large arsenal of weapons held by most of the people in the nation would deter any would-be tyrants from even attempting to impose a tyranny in the first place.
Any other benefits of the 2nd Amendment, such as having a large, trained pool of people able to shoot well in order to oppose any invading army, protection against criminals (including terrorists), and various gun-related sports - these things are simply incidental to the main purpose of preserving the Constitution and the Republic, or of taking it back if it were ever lost.
That being said, you are correct that the police, armed forces, National Guard and various intelligence agencies cannot protect us against every threat. Having citizens bearing arms in the streets, in airports, sports stadiums, etc. would do lots to deter both crime and terrorism, and to combat them if such deterrence failed. I, along with many others, believe that the WTC towers would still be standing if a few passengers had been armed, as was legal before the early 1970's. Gun control has claimed another 3,000+ victims, and is on its way to destroying more of our freedoms.
Remember too that in their day outposts of the empires of Great Britain, France, Spain and Russia were on or near the then-existing or future borders of the new nation, and if those countries seemed complacent or benevolent in their outlook toward the new American nation, those wise men knew that such things could change.
And now along our southern border, we face just such threats from those Mexican reconquista forces flooding that border with invaders. Those founding fathers gave us the tool with which to cure that problem, if today's later Americans will but make use of it.
It looks like it's coming to that.
-archy-/-