Never, sorry. Bitcoin is new technology and there is always the possibility that something will come up that hurts the protocol.
Don't get me wrong, I love bitcoin, but never can be sure.
While bitcoin's protocol can be changed. You can't change it on the whim by bitcoin developers, you will need consensus of majority of bitcoin users for that change.
An I can agree with you that for now there's no guarantee that if you invest your money in bitcoin you will win the future... There is too many positive news that have no effect on bitcoin imo.