They are giving I Owe You papers not numbers, this "I Owe You" paper MUST be paid if requested, the thing is that by losing money they because of demurrage, they lose the power of give people their "I Owe You" .
Sure, but they can simply count on the fact that only a small number of people ever actually request to withdraw real bitcoin, and an incredibly small number would ever request to do it at any one time. Hell, even if they just implemented a delay of 1 hour for withdrawals it would give them plenty of time to buy more bitcoin elsewhere and transfer it in to their own accounts to cover the withdrawals and continue running their fractional reserve system with very little reserves.
Also, you wouldn't do it with already existing coins, and if wanted to be done with already existing coin you could do some fork of a existing coin you want to implement this feature at.
I would imagine you would have a very hard time convincing people to move away from bitcoin (or any other coin) and to a forked version which is exactly the same except you lose a portion of your holdings over time.