As a company I don't want to be responsible for offering paid downloads or virtual gaming goods and access to services eternally.
How can I tell my customers that virtual goods expire? That they lose value after certain time?
I think this will best be solved through a combination of text based time remaining warnings - i.e.
My Download files ( Link will expire on 01/01/20013)
And appropriately designed warnings as the user gets close to this date. For example when returning to your platform a portion of the UI could display a red/orange/green background behind each available asset depending on when they expire and how close the current date is to that point
Most sites that serve up digital goods give you X amount of download links OR if it's a subscription / access based purchase they should know up front that their offer is only valid for "1 year from the purchase date". Once they activate their account you should send them an e-mail that thanks them for activating and lets them know how long it lasts.