If longbets.org was really serious about long-term bets, it wouldn't be restricting years to only 5 digits. Haven't they heard of the Y100K problem?
But luckily they used 5 digits. 5 digits should be enough for anyone.
[1] https://en.wikipedia.org/wiki/Timeline_of_the_far_future
The tricky part is coming up with a clear agreement about what "cure" and "death" mean here. But there's a lot of good discussion around e.g., the extropians, so I think a solid bet could be created.
Or do you mean for your own? This is between you and the person you bet with. If you have a firm opinion, definitely find somebody on the opposite side and see if you can put together a bet.
Payout are to the bettor's selected charity. Or, if that's gone, the closest one we can find.