Default instance blocks should largely replace defederation
Since what content users might want to see is quite unlikely to match which servers the admins tolerate, choosing instance on the Fediverse can be quite complicated, which is inconvenient and off-putting for new users.
For this reason, and simply that the Fediverse is stronger united, I believe defederation should ideally be reserved for illegal content and extreme cases. If Fediverse platforms would allow instances to simply block the rest for users by default, the user experience would be the same, unless they decide otherwise.
Content isn’t cached unless someone follows it anyways.
And I’m not sure what you mean with that latter part; what difference would this make in what content admins can see before they cast their judgement on a server?
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).