I may well be over thinking this, but, by effectively unbundling the flights from being a booking, it removes the onus on Regent to ensure that the passenger to get to the ship, home, or wherever.
Think back to the beginning of Covid when the cruises were cancelled and passengers who had booked the fully packaged trip and were stuck had to be transported home by Regent.
in the UK, it may also remove some of the protection that booking a package holiday provides.
LP