I think most people choose which cruise to take based on far more than just a dress code (price, itinerary, entertainment, etc.). Besides, with the exception of NCL (yuck!), all the mass-market lines have formal nights so it's not as easy as "if you don't like to dress formal, choose another cruiseline".
But all the mass market lines now also offer casual alternatives so there is no need to avoid cruising if you prefer to remain casual. And the main dining room isn't as formal as it used to be so even if you dine there, you don't have to dress to the 9's in tuxedos or ball gowns. Something far less formal will do.
I really wish that formal night would be done away with, but I'm not going to stop cruising or stop cruising a particular cruiseline because of it. I usually make do by using the casual alternative, however if I do go to the dining room on formal night (usually because my traveling companions prefer it), then I make do with a dressier business-casual outfit such as a pair of black slacks and a pretty blouse. As I've said before, what I choose to wear is really between me and the cruiseline, and since I have never been turned away, I have to assume that they find my appearance acceptable.