There is no cruise line that I will sail exclusively nor is there a cruise line that I will not sail.
Of course I have preferences but a cruise, to me, is far more than just a ship or cruise line. It is the itinerary, the logistics of getting there, the cost and then the ship and line that are the equation that decides if I cruise or not.
I have never been on a ship that was SO bad that I would not sail it again if the itinerary and price were right. My least favorite ship, so far, has been Norwegian Epic but it was by far not bad enough to say. I won't sail that ship ever again.
There are also no "loyalty" programs that have benefits so good that I will pigeon hole myself into their ships and itineraries. Overall I haven't seen any loyalty program, including the luxury lines, that would even make me put the loyalty program into my cruise equation. Unless the past passenger discount was REALLY good.
I believe there are too many places to see and too many ships to sail to allow me to narrow my world just to sail on one cruise line.