It is not just young people who no longer want to follow formal wear specifics. I'm well into my 2nd half century. Done plenty of true formal wear for one lifetime. I still dress up, but not like when I was younger.
You're absolutely right. In many cases, it's the "seasoned citizens" who don't want to dress up for formal evenings!
That said, I think there's good demand for an "all casual" major cruise line, and I'm surprised that neither Carnival Corporation nor Royal Caribbean Cruises Ltd. has moved to introduce such a product. I would like to see the latter split the Royal Caribbean International fleet into two lines, one of which would maintain the present name and present standard of dress while the other would introduce a new brand and an "all casual" dress code (that is, NO "formal" evenings). I think that the response wo the latter would be very strong.
Formal wear trivia...the "no white after labor day" also used to apply to tuxedo dinner jackets. Men wore black from Labor Day until Easter, then switched to white dinner jackets. I actually enjoyed one aspect of all that. When Easter came, the white patent leather shoes came out of hiding and that meant that winter was over...even if we got more snow and had to go back to winter shoes and clothes.
The rule about when to wear white probably originated in England, though the "Labor Day" cutoff proably is New England adaptation from a British holiday occurring in early September. In any case, the growth of the Empire clearly made it locale-specific. White is always acceptable in the tropics, and the season grows is longer as one gets closer to the tropics. Of course, a black dinner jacket is always acceptable anywhere at any time of year.