While this is true, the majority of cruises are sold through a TA. Also refer to the article I mentioned about.
If one cruise line increases fares by say $250-$300 per cruise, it makes them less competitive.
The majority of cruisers may use rewards programs but aren’t loyal to one. That $150/pp price hike on travel search engines x a family of 4 will 100% reduce bookings and profits for any line that attempts this.