Comment by goblin89
The marketing move of offering an unlimited plan reveals that storage and traffic are not that expensive and someone made a choice that light users will subsidize heavy users. With that, hiding your data from you and subsequently deleting it, at least without first encouraging you to download it within some post-downgrade grace period, would be a choice, not necessity, and is user-hostile.
If it is an actual necessity—a service chose to market an unlimited plan to attract more users, and then realized they are losing money on storage and traffic so much that they would unapologetically burn bridges with existing users who showed themselves as willing to pay (who maybe needed to downgrade temporarily for whatever reason) with the above move—and yet their strategy is apparently to keep offering that plan (in hopes to turn things around with more light users joining?), I would question whether that service has serious issues with even medium term planning.
No matter their actual costs to provide the service, I'm struggling to see why they should not immediately delete all of your stored files upon cancellation of the storage service.
They are a European company, so you are the customer, not the product and recipient of subsidies. They use less manipulation and dark patterns than an equivalent American company.
You pay, you get service. You don't pay, you don't get service. If they can't bill you, they should try to communicate with you for a few months before treating it as a cancellation. If you cancel, then your choice is clear and you should expect your service to be immediately terminated at the end of the current billing period. If their service is storing files for you, termination of the service means deletion of the files.
There is no need for a grace period when you knowingly and voluntarily make the decision to terminate a file storage service.