I love your outlook, because you are suspicious of most people just like I am.
But in this case, I think they kept formal nights because they help to generate revenue, but without a doubt it has been a feature of cruising since ocean crossings first came in to service, so it isn't just a gimmick. Many people would protest if they were taken away.
However - if they detracted from revenue there is a good chance cruise lines would get rid of them, because they are very revenue oriented.
But in this particular case - I would put the "blame" more on the Steiner's hard sell. They are notorious for using every little trick in the book to generate extra revenue.
It is tough for the cruise lines because somew things have degenerated to the point where they can hardly make money - like photography. I remember when most people bought a stack of pictures onboard. But now with digital cameras people buy less. However they can produce a print for far less, also, and they sell them for 500% more than say 10 years ago.