The way I read it was that granny squares saw a cycle of popularity, rather than crochet in general. 40 years would be a pretty normal length of time for something to be cool, then dated, then cool again.
Personally, I learned granny squares and mile a minute afghans from my mother. Until the last 5 years or so though, I definitely saw them as old-fashioned and out of date. Now they're coming popular again with different color combos and I'm coming back around.
Maybe the latest peak is pushing out due to having kids later in life (i.e., grandparents now 50-60 years older than grandkids, rather than 40-50)