I think the term "you get what you pay for" holds true to a certain extent. But, there's all sorts of things that factor into it as well.
I think it's quite understandable (and common) for someone who goes on a cruise, enjoys it, and feels like they got great value, to cruise on the same line again. A lot of people feel if they're happy with their brand choice, why risk their money on a more expensive brand that may not suit them.
If someone buys a Honda, and it serves them well, and eventually needs replacement, I think they are probably more like to buy the same brand, than they are to buy a more expensive Mercedes... just because it should be better because it costs more.
I also don't think it's at all uncommon for people who find the cruise company product that suits them the best, to decide that company will be best suited to everyone. And that anyone unwilling it to try it is making the wrong decision.
The brand loyalty is what each of the cruise lines rely on, and they attempt to build on that with their loyalty programs. And when you talk to people about why they like a loyalty program, they will almost always say, it's the "value" they feel they receive from the perks.
I've personally been able to sail many different brands over the years, because of my position with CruiseMates - from mainstream to ultra luxury. Of the mass market lines my current favorites are Carnval and NCL (yes, above RCI).
By no means do I want to insist they are the best for everyone. It's just that their style suits me.
I get many, many people asking me what is the best cruise line is. If honest, I'd always say Crystal or Siversea. Of course, many people say "they'd love to try them, but they are too expensive".
I can never look into someone's "wallet". and tell them they can afford it, because they'll get better value, and they should give it a try.
If someone can afford it and wants to try many different cruise brands, I say great! It's wonderful to have all sorts of different experiences. But, if they found a cruise line they know they really enjoy, I'd never want to suggest there are "better choices"
if they'd just listen to me, and spend more of their money.
If anyone is that sure, they should buy the ticket for that person to "see the light"