That is pretty much what the article says. It is not so much about less demand from the pax rather than airlines phasing it out due to barely any difference between first and business.
The super rich fly private and most companies will book business, nowadays even for senior management.
Thus in some way, there is a decline in demand which is due to a decline in differentiation.
However, to mimic it in the game, the easiest way might be to have first class demand decline from the passenger side and a shift towards business or "premium business" (though just business would avoid any additional class).
That would also allow for more business/prem. econ/ economy configurations without any significant drawback in the years after 2005 and onwards.
Only trying to find a way implementing this trend with as little impact on the porgramming side as possible.
Hope that clarifies a bit