I dont operate a relay, but if I did I'd be thinking about the growth in usage on data transfer, memory usage, connection costs etc more so than storage.

That said, you have a good point about continual growth of that storage. As a user, I generally don't need the ability to fetch too far in the past for all messages, and actually don't mind things expiring/deleting off on a rolling basis. E.g., 90 day old would likely be plenty. Useful features for paid services may be the ability to bookmark/flag/save N number of events on the account to be retained indefinitely, and users may be able to increase the max that can be kept by paying more on a recurring basis.

I'd expect there to be relay services that arise for "Compliance Reasons" to capture and retain everything for business/government for like 7+ years or some such, but that may be archived somewhere and not necessarily readily available to retrieve via protocol.