I got rid of most of my cassette recordings, but am keeping the factory tapes and all sealed blanks. I have been to a number of estate sales, garage sales, antique stores, etc and used cassettes, even sealed and unrecorded just don't have any value. I am doubtful about the future value as well. Cassettes just don't have the "cool" factor of vinyl, or RTR tapes, IMO. Many of them don't sound very good either; being either poorly recorded, or just plain worn out after years of bouncing around in very hot or cold car interiors.
While I continue to record on RTR (and CD), I have not recorded a cassette since no longer having a player in my car.