I definitely have become more informal the more I cruise.
My first cruise, I wore a tux on both formal nights. For the other "casual" dinner nights I wore a shirt and tie and also a jacket.
My second cruise I wore a black suit, and just a shirt & tie for dinner.
My third cruise, I wore a black suit and only wore a tie on occasion.
My fourth cruise I got married on board, so that was different and I did wear a tux on formal nights, but no tie for the other nights.
My 5th-6th cruise I only wore a suit formal nights, and just a shirt for the casual evenings and even wore polo a few nights.
My 7th cruise we only attended 1 formal dinner which I did wear a suit. The other nights, I just wore either a polo or a button shirt with no tie.
I definitely have become more and more informal. My next cruise in April I do plan on attending both formal dinners so I will wear a suit for those nights. The other nights, probably just a shirt and no tie.
I'm not sure why I became more "informal" through the years, but it does seem to be the trend though. I never wore jeans in the dining room and doubt I ever would.
I wonder if part of it might be the baggage fees airlines are charging?