I got a gift of No. 5 when I was 18, and I did wear it, and I did like it, yet whatever attracted me to it at that young age vanished.
It was a coming-of-age experience, and I think you have noticed something worthy of comment. During WW2, No. 5 was one of those fabled "French perfumes" that soldiers brought back from the European theatre of war. It had a very strong brand that has continued to this day, undiluted, whereas the same cannot be said for the competitor brand Guerlain, which has not retained as strong a presence in the upscale market. Caron has, but Caron does not enjoy worldwide recognition. Chanel No. 5 is the Rolls Royce of the perfume world and has since its debut been sold on the basis of its sophistication and the premise of buying a bit of elegance and tasteful chic.
In many ways, this scent remains a rite of passage. It is still synonymous with class, which is a quality that is sorely lacking in modern commercial perfume releases, which are all about democracy and lowest common denominator (hence vanilla and fruit) and the financial bottom line.
Moreover, as a baseline for classic perfumes, it stands unchallenged. Even if the giftee does not care for the scent, the message is not weakened. The giftor has chosen something that women universally recognize as a luxury good, and in turn it addresses both the good taste and the consideration of the purchaser.
No. 5 succeeds on three levels. First and foremost, it is the nonpareil of aldehydic, formal florals. Secondarily, it succeeds as a brand standing alone from the Chanel label in the way many luxury cars do (Silver Shadow, etal) Lastly, it is a way to purchase into a presumed lifestyle.
Had the person purchased Allure or Chance, the message would have been weakened. Understanding this young man's ignorance in perfumed matters, the clerks were correct to select something that would transmit a very specific, wordless communication.