From 90% already in circulation, about 3.7 million Bitcoins are reported as "lost"...
What are your prospects for the next few years with regard to Bitcoin?

Andreas Antonopaolous stated in his video:

“The cryptographic keys and algorithms used in Bitcoin do not last forever [anyway]. At some point, the elliptic curve digital signature algorithm (ECDSA) and SHA-256 will be obsolete. Maybe in ten or twenty years; certainly less than fifty years. They will no longer be strong enough to protect coins that haven’t been switched over to… a stronger cryptographic algorithm.”

Moreover, Antonopoulos added that the Bitcoin code needs to be recycled from time-to-time into better cryptographic algorithms; people with enough computing power should/could break keys and retrieve the lost coins. He also added that recovered BTC could act as ‘bounty’ for dedicating their resources.

“Essentially, it creates a bounty for testing the system, and rewards people who [exercise their skills].”

https://www.youtube.com/watch?v=IYtAevJIy5E
Minute [4:20 - 7:50]