You're right, Kuki, with a dining room split into a dressy casual side and a formal side, there would still be some who showed up in cutt-offs and t-shirts. But they are likely the same people who are doing that now. Look at it this way, at least they would be on the casual side instead of sitting at YOUR table.
You asked why those who don't like formal night don't choose a line like NCL or Windstar? I can't answer for everyone, but for me, it's because dress code is only a small portion of the cruise experience and, unfortunately, NCL and Windstar don't offer the rest of the package that I'm looking for. Why don't you choose a line like Cunard where you know the dress code will be enforced?
They could (and they should and probably already do) turn away those obviously flaunting the dress code in cutt-offs and t-shirts. I have honestly never seen anyone show up dressed this way and I doubt this is what most advocating for a more casual dress code have in mind. I rarely eat in the main dining room, but when I do, I wear the same regardless of what night it is: nice slacks and a pretty blouse or sweater (the same as I wear to work everyday). I figure if it's good enough for work, it's good enough for my vacation. If the cruiselines started turning away people like me (nicely dressed but just not formal), they would be alienating too many people. Society has become much more casual than it was 40 years ago. What worked then, won't work now.
I predict it's going to get worse before it gets better. With each passing cruise, I have noticed the buffet getting more and more crowded with people who don't wish to participate in formal night. Apparently, judging by the complaints on these boards, there are many more people who don't dress but show up anyway. The cruiselines are going to have to come up with better alternatives to accomodate these people. Not everyone is as happy with the buffet as I am. Having large number of people feeling like they are being treated like 2nd class citizens is not going to work for long. It was fine when there were just a few, but their numbers are growing (and fast!).