In the long run it would be strange for this to be the case, no? What are the economic forces that give rise to this? Maybe sometimes recessions affect poorer people more, and first class tickets smooth out revenues?

But in the long run, we've seen a steady worsening of the economy experience as airlines have invested in improved first class experiences. In the long run it's simply impossible for a firm to serve poor people (they have no money), absent interventions that allocate buying power to the poor person, or others that force the firm to do so.

I think the worsening economy class situation is simply consumer choice. Discount carriers offer worse service for a (sometimes minimally) lower price. People pick that, so other airlines do the same (or they would loose customers).