I'm wondering about one thing... If you take a look on majors' longhaul fleet cabin configs, for example LH or BA, you will notice that F+C cabins represent about 25-30% of total seat number.

If I would exclude frequent flyers, and just talk about "normal" people, vast majority of them never flew loughaul in F or C. Talking to people I know, even many of them can afford it, they still think it is a waste of money. Arguments would be like "look, for EUR 3-4k difference I can stay for a week or more in a luxury 5 star hotel, why should I value 10 hours flight same as a week stay?", or same thing about cost of dinner in michelin-starred restaurant vs C ticket etc. And many of those people really stay and eat there, but still not flying F or C. Seriously, I would say only 10% or less people I know ever paid for premium class, and I wouldn't complain about social level of people I know.

Now, I'm myself never flown paid F (only got op-up a couple of times), but most of the time I'm flying in paid C. Very rarely I can see C class more than half full. F class is mostly empty. Flyertalk is full of treads that travel policy is not allowing to travel in the front.

Judging by cabin configs, airlines assume that 25-30% of people are travelling in premium cabins. My reality check says it is 2-3 times overestimated.

What would you say?

I would say that 83% of the people who toss around speculative statistics make assumptions that are incorrect 97% of the time. Including me.