I was the original poster of this subject back in March. I just made final payment this week for the 2025 trip. While I contemplated cancelling, I had discounts and vouchers totaling over $3000 committed to this trip. I was "pot committed" so to speak. I also purchased Viking's trip insurance, about $1700 on a $13,000 trip. There were other policies that were less expensive, but Viking's was the rare policy that let's you "cancel for any reason". I felt that was worth a couple of hundred dollars extra.
I have learned some of Viking's reasons for the extraordinarily early full payment. As part of their business model they require full payment so they can staff their ships, book personnel for excursions and lock in provisions at today's rates (or as much as possible). So for them, it's an inflationary control. Apparently early, committed (paid in full passengers) has contributed to their success.
From the customer's viewpoint, by booking and paying so far in advance, you MAY be locking in a fare that is less expensive than it would have been had you booked it six months ahead of sailing instead of sixteen months. Combine that with the 3.3% they knock off for an ECheck payment, or whatever you may receive on your rewards credit card, and perhaps it's a wash on the interest you would have earned on that money....perhaps.