(A less evil approach that might still lead to reduced costs would be detecting old, unaccessed data of sufficient size and flagging it for users, with a small refund or service discount if they purge it. Though that assumes you have 'power users' who are storing massive amounts, to where the savings in storage costs would be worth it)

(And if you don't already, I would also consider making it so items that are in the trash for some period of time, say 30 days, get deleted automatically as well, possibly with a reminder email a few days before)

(And lastly, depending on user profiles and usage, incentives around reducing resolution/quality of photos and video, and automating that in the app as part of the sync process, might provide some opportunities to reduce costs of storage > the lost revenue of cheaper plans.)

"We used advanced machine learning algorithms to predict which users will need to retrieve which pieces of data in the future, and silently delete everything else."

What about storing the older things in some slower backup service that's slow to access but cheap? If the user eventually accesses those some day you would kick of some background job to get them fresh again. Not super UX friendly of course, but could reduce costs. Or is this already the standard thing to do?