Well, some people just cannot get over the belief that what they have been eating is the perfect diet and everything else is FAD.
I deal with this every day. At work, when I strictly refuse to eat sweets or anything else that does not fit into my diet plan, all I get to hear is "itni kam umar mein ye haal hai toh aage kya hoga. Hamne toh khoob khaya hai tumhari umar mein". I get surprised not because of what they say, but because of what they believe. Everyone who says this to me is suffering from obesity/arthritis, blood pressure, diabetes and cholestrol. They are heavily dependent on drugs. Still they believe that what they are eating is good for them. I see all these people devouring sweets, biscuits, samosas, pastries and what not almost every day. I so wish they would change their mindset.
Younger generation is quick to adapt to changes. I have altered my diet completely and as you said, I am never going back to the so called balanced diet ever again.
The problem with most elderly or middle aged people is that they do not want to accept any change. Yes, they have years of experience but I feel they should be open to changes also and not be so rigid in their mindset and views.