I wonder if the "change" is due to the change in how we book our cruises? I remember years ago, when one had to book through a travel agent (IMHO) one got better service from that travel agent.
I remember when we booked our first cruise, getting detailed instructions from the TA on the dress code, what to pack etc. Now, I sometimes book myself on the internet, and sometimes go to a TA for OBC or better deal. The "interactions" with the TA are quite limitted. In the last year, I've booked two cruises through TA's and neither one of them made ANY mention of what to pack or what to wear ....
Is it possible that travellers who've just started cruising in the last couple of years, have not been "educated' on cruising dress codes, .... pack what they do ... wear it .... get away it ..... and then the next person says, "there were so many men on formal night not wearing a tie, I'm not wearing a tie anymore either" ... and so it spreads?
I have no problem following dress code, if I know about it in advance. If booking a Cunard, meant that DH had to bring a tux ... then we'd bring a tux. If no one told us ... he'd just pack a suit.