If you have Costco as your agent, they base the insurance on the cost of the cruise only, but cover the flights and hotels as well. And you get preexisting coverage if bought within 24 hours of final payment. So, your scenario isn't always right. You just have to have the right policy.
I'm sure there are limits on the Costco (or other) insurance policies too. If you leave your house and fly to WDW and stay there for 2 weeks before going on the cruise, and all you've bought is insurance for the cruise, I doubt your 2 weeks would be covered. There has to be a dollar limit or at the very least a time when the policy kicks in, like maybe 24 hours before the cruise. If you fly 1st class and only pay for the coverage for the cruise, again I think there will be limits on what you can recover.
No policy is going to cover everything, so just know what is covered. The OP gave a good warning.
Nancy.