I don�t get the cruise industry but I�m guessing that this was started well before the pandemic. The size would limit ports but I guess when the industry was going strong it made sense to someone.
The more passengers they can accommodate, the more money they make. That's the business case. But if the company that commissioned the ship can't afford to finish it, it's gets melted down.
9,000 passengers is just too ridiculously big for a cruise ship, not to mention there would be a couple thousand crew as well.