What op said still stands: if only one of your users follow a high-traffic, heavy-content /c/, then the server is caching all of that content for one person.
E.g., there’s this great bot on Mastodon that posts random fractals, and the highest-voted ones “breed” to create a new generation of child fractals. The bot posts a static image and an animated movie of each new child every 4 hours. The images are ca 5mb each; the movies are between 20 & 40mb ea. That is, on average, 210mb/d, or 1.4gb per week. That’s a lot of data. You might, as an admin offering a free service, not want to have to pay for that much storage just because one or two users are suscribed to /c/flamereactor (“FlameReactor” is the name, so you can find this mind-blowingly awesome bot). There’s also bandwidth considerations, both on the pull and when users request the content.
I like the idea, though, and will suggest a tweak, tried and true from Usenet days: provide the ability to unblock to only paying users. It’d give admins control, plus money to offset storage costs. Maybe provide three options to admins: full defederation; auto-block with any user able to unblock, for odeous but low impact sices; and auto-block with unblock for only users in some group - close friends, paying users, whatever.
Lemmy could also transcribe content into links back to the source, but that’s just punting the bandwidth costs onto someone else, and I wouldn’t be surprised if this is frowned upon within The Federation (although it’s common practice with Reddit and X(twitter) content).