Out of fashion food?

The other day I was reading comments on one of my favourite blog (she doesn't need publicity, but here it is!) and someone wrote: "stop with avocados, they are so seventies" (in the meaning of obsolete). Well, I found that comment quite strange.
Let me explain myself. On which basis can you label a certain food out of fashion or obsolete? This concept never even cross my mind, and it's more then 10 years I'm seriously around food, 5 of which professionally. How can a food, something nature gives us, be defined obsolete?
There are a lot of lost vegetables or cooking techniques (my father in law grows many légumes oubliés, like 10 different variety of tomatoes, 5 or 6 different pumpkins...), lost during the centuries, due to famines, weather conditions, wars, drastic change of diet... But can someone decided that a vegetable, a fruit can become obsolete??? I personally do not think so. I think that everything is good, and with few basic skills everything can become an extraordinary mouthwatering dish.
I left a "talk" here and let me know, in comments or e-mails, what you think... Your opinions matter to me... As long as they are on the same track of mine! (;-D Just joking!)