My son is usually the only platinum in the teen club. There's rarely any kids that are even Gold. I think most of the Pearl levels will be empty nesters. We will be Pearl, but my youngest is now an adult so I'm not sure how often will cruise Disney anymore.
Yes, I'd expect most Pearl members will be people traveling without kids (either because their kids are grown or because they never had kids) who like the comfort and familiarity of Disney cruises. There will also be a growing number of people who reached platinum (or near-platinum) cruising with their parents when they were kids and are now cruising with their kids. Based on reactions to the Wish saying it's kid-oriented (and the addition of a dedicated space for the 18-20 group), I'm guessing DCL is going after that second group.
People who have already decided to cruise with DCL more than 15 times are unlikely to think "I was going to stop after 20, but now that there's a new tier, maybe I won't." They are already repeat customers. However, people who enjoyed Disney cruises with their parents, have taken their kids on a few Disney cruises, and are debating whether to stay with Disney or shift to Royal/Norweigian/Carnival as the kids get older might be swayed if the benefits of staying with Disney are appealing.