I have a theory that Carnival Corp. chose Holland America to pair with Seabourn because that is their most senior-American based clientele - but that clientele is aging and HAL really needs to attract new blood.
HAL has needed (and been trying to) shed the image of an old-folks home cruise line for years now (and they are getting there). But my guts tells me that you will soon start seeing far more former HAL clients being moved over to Seabourn. I wouldn't be surprised to see announcements that HAL is retiring some of its S-class ships soon; Statendam, Maasdam, Ryndam, etc.
These S-class ships are smaller and quiet and generally do longer and more exotic itineraries - meaning the clientele is largely similar demo to Seabourn (in all aspects except perhaps income).
The new Seabourn ships are somewhat unique in that they do a fair number of 7-day itineraries - even in Europe, and this is actually a pretty mainstream thing to do. This is a good way to get the HAL folks to try Seabourn.
You can get a 7-day med cruise on Seabourn Odyssey for $2999 (295 sq ft suite) or a 7-day med cruise on HAL for $2699 (243 sq ft suite). With that kind of comparison, I think the upsell to Seabourn would pretty darn easy - especially with the inclusive nature of Seabourn cruises. They come out to the same price for all intents and purposes.
Yachts of Seabourn - Luxury Cruises
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