Presumably people have brand loyalty because they think they're recieving a good product they like. That's not a dark pattern, that's good!

Or maybe I'm being too idealistic as to what drives "brand loyalty".

If the "better" really corresponds to the "more", that's not exploiting brand loyalty. That's just when a brand is an honest signal of the branded product or service. Exploiting the loyalty is when they cheapen the product or service, but keep the pretense of the brand. Singer sewing machines and Pyrex glass cookware come to mind as prime examples, but I'm sure there's many, where someone buys the company for the brand, and then drives it to the bottom for quick profit.