Whilst agreeing that times and tastes have changed, I do think there is, and probably will be, a place for the traditional aspects of cruising on most lines for years to come.
We just sailed on Sky Princess and on formal nights the majority in the MDRs did seem to follow the dress code, and I although I have not sailed on RC, Celebrity recently, I imagine that they also have a high proportion of traditionalists in the MDRs on formal nights.
I agree that dinner jackets are not regularly worn in public, but I think this is probably why they have remained popular on cruises, and probably will stay popular for years.