A cruise line's best bet is to incentivize a pre-cruise negative test result somehow. Either via a perk onboard or with some CWC protection that those not presenting a negative test wouldn't get.
I've seen people order blue cheese stuffed olives in a martini before. But that was only after a gracious tip to get the bartender to stuff the olives.
Yes, you're right. Norovirus, which is extremely contagious and very easy to swallow a ship. So that makes sense.
But for any airborne upper respiratory virus, not as much.
The interesting thing here is if they're not paying for your quarantine anymore, then there should be no more quarantine rooms either. Just have it like any other illness on board.