It was easy to grow accustomed to Regent's "elegant casual" every night. On some cruises longer than 16 nights, they will have a "formal optional" night wherein I would wear a tux so that I fit in with those that enjoy the more formal atmosphere. I do take a couple of sports coats and a tie for Seven Seas Society dinner but otherwise just wear slacks and long sleeve shirt.
You shouldn't mix formality up with luxury. You can have luxury and formality together (e.g., SIlversea) but they aren't the same thing.