I think when their is already a coin age and the transaction is bound to a block, the coin age cant be faked. Which means btc couldnt expire before 99 years. So it would be safe to assume that btc that lie 99 years at one address are abandoned. I dont see a reason why there still should be an owner after that time.
So this way the available amount of bitcoins remains stable plus, when all 21.000.000 btc are mined the network could held stable by let these btc mine again.
So i dont see much of a risk at this. I think the more that it could help stabilize the network because miners could find a lost treasure this way, keeping the network running after all is mined.
Because i fear, when all is mined the network will be in trouble until fees for transactions are high enough that miners work again. This could only help a bit of course.
Anyway... youre right... in 99 years bitcoin probably is gone. Or its a sunsystem currency...