The problem is that you have no idea when or how the whale acquired their XC. What if they bought all of them at a price higher than the current XC price? Then they haven't technically gained anything from XC to be able to give back to the community.
I agree that BTC is best, but what if the whale only owns XC? They would have to convert their XC to BTC to pay their share of the bounty. Fast forward one month and no one can break the code and as you say, the price of XC doubles. Now the whale has to buy back their XC at a higher price which means they lost half of their XC, not gained XC.
Those are good points. It might be best to just try to reach a certain amount of Bounty so with community offers in BTC and see what we reach. How those BTC will be earned will be up to the individual Donors and after that, the Bounty can be Kept by the XC team and redistributed in BTC to each one that have donated with the same amount that they have donated.
Edit: Even if the 1.8% balance will decrease in value as they might loose some XCs if XC goes up in value when they will need to buy back the 98% of what they already own will gain value from the Increase balancing out the XCs that they will loose. This is another point to consider.
For the other point if the Big Whale bought on an higher price then what it is now or it will be at the time of the ann for the bounty then it might want to reconsider how much to donate. the point above will still be valid as 98% of what he hold will get the benefit from the gain in value after the Bounty.