The idea of an 'instance' is not really clear. If you want to run a social network on ATProto, you do need the relay and app view, and those are indeed heavy requirements, but the architecture is not the same, and it depends what you're actually trying to achieve.
If you just want to use ATProto for your own small app, there are ways to do it on more constrained hardware. The more usage it gets, the more it scales up.
> On ActivityPub and Nostr you develop all kind of applications already, it is not only Twitter clones, but also Instagram clones, Reddit clones, on Nostr there is even the resurrected Vine (https://about.divine.video/).
Same! You can explore the (unfortunately named, given the point I'm trying to make :)) 'Bluesky directory' to see apps which use ATProto: https://blueskydirectory.com
Note that most of these have nothing to do with Bluesky.
> And it is not trivial to switch from the Bluesky did:plc (which you cannot run in write-mode on your own and need to use the official Bluesky owned) to did:web, which you can run on your own
That's true, but it's not trivial to run your own Mastodon instance either...