I can believe that the number of French cuff shirts sold has declined, but I'd attribute that more to the increasing popularity of more casual dress (in both business and social contexts) over the past couple of decades. French cuff shirts were a more reasonable purchase when, say, a man was expected to wear a suit to the office 5 days per week, than they are when that same man is expected to dress in business casual fashion. Since many men who would consider wearing French cuffs with a suit, would not care to wear them with chinos or jeans.
Now, having long fingernails can make it more awkward to use a keyboard. Yet the number of nail salons in the US has increased dramatically from what it was 20 years ago. Go figure.