This does seem like an amazing fantasy, but a fantasy nonetheless. Amazing that it's even potentially a thing.
I never would have thought any cruise company could offer it. There's a reason most cruise ship vacation packages are 1-2 weeks, though there's a few that are 3 weeks.
Because okay, back to the real world. Forget that it starts off at $60K (which is for an interior stateroom that would get old fast), so it would be more like $70K+. That's actually the smallest hurdle that's the easiest to get over. It's the same reason why some individuals can buy late model luxury cars, or big mansions.
People have to go to work. Even if you're lucky enough to have 5 weeks of paid vacation time, 5 weeks is not even 2 months, much less 9.
Retirees also have time obligations as well. Do you really want to spend all the various holidays alone? And children have to go to school.
Okay, suppose you're single, you're estranged from your family so you never see them over holidays, and you have a job that's 100% remote/telecommute. Too bad Internet access at sea is generally insufficient and not good enough for that. Even if it was, there may be legal and practical barriers. Legal because you may only be able to live and work from certain areas, or may only be able to work from an alternative area for a couple months at most. Practical because time zone differences are a thing. Political because even if you can overcome the legal and the practical, imagine the intense anger and jealously of your coworkers.
Sorry to burst everyone's bubble. But the average person could never hope to do this, or vast majority of the bottom 99.9%. Probably not even most of the top 0.1% as most are insanely busy, running their enterprises.
Maybe if you're a young person just out of high school who comes from a fantastically rich family and you take a "gap" year before college, a distinctly European concept. That's the only other demographic I can think of that could possibly take advantage of this.
9 months