I meant relatively primitive where there were not as many things to change and the skill of the user was the primary factor as opposed to modern warfare. For example in modern tank warfare the technology is just necessary to gain victory and the experience of their users has little to do in a fight of (for example) T72m vs. M1A2.
Secondly I agree with you that there are a lot of records to confirm that there was a lot of work put into longswords. I believe that the best ones were made in Italy. Am I right?
Yes but with your example the rapier was specifically made to slide into the gaps between plate armour, hence I am not sure whether the factor of armour itself is such a problem in this scenario.
Don't forget that the user of a longsword would also have a shield. As far as I'm concerned there are no gaps in a shield.

Also you need to remember that it isn't that easy to just hit whatever part of the soldier you want because he's going to move, he'll protect himself with a shield etc. and will also try to attack himself. In this occasion the user of a rapier would have to dodge the longsword as he wouldn't have much choice. Your 'plan' of striking in gaps is good but no plan survives first contact with enemy (some of them actually do!(not as often though))
So, as a final judgement I shall conclude that most is up to the skill of the user. Full stop.