So I will just say does everyone test each morning before leaving their house? Its the same concept with getting on the cruise except a larger percentage of people you see will be vaccinated, more people will be wearing masks, better cleaning protocols will be had, and its likely "safer" than a common public environment you will find in the US.
Additionally this testing is something Disney should have done from the start (instead of it being unvaccinated only), they should have required COVID19 insurance since the start (to cover quarantine or private transport home), and they should have worked with a private airline or car rental company to have on standby to refer you to if you test positive.
Getting on a cruise comes with an inherent risk you will get COVID. If you want to avoid that risk then you should be cancelling. Additionally the riskiest action everyone is going to do is get on an airplane roughly 6-36 hours prior to getting on the cruise ship. Which means they won't test positive until after boarding but will possibly be contagious while still on board.
Final note Disney should be push everyone to test 72 hours out from the cruise as well. NCL (if I understand correctly) as an example will cover all quarantine costs if you test 3 days prior to showing up and test positive.